The film's title is open to interpretation. Robert plays a Chopin "piece" on the piano because he says he learned it when he was young, and it's "easy" to play. He only plays only one other piece in the movie, but there are a total of five classical pieces played in the film:
Frédéric Chopin - Fantaisie in F minor, Op. 49
Johann Sebastian Bach - Chromatic Fantasy and Fugue
Wolfgang Amadeus Mozart - Piano Concerto No. 9 "Jeunehomme"
Frédéric Chopin - Prelude, Op. 28, No. 4
Wolfgang Amadeus Mozart - Fantasia No. 3 in D minor, K. 397
Another interpretation could take Robert's misogyny into account and refer to the three women he fools around with as "pieces". These women, in addition to the two piano pieces he plays, brings the total to five.
Another answer: the original screenplay (by Carole Eastman) included a sequence through the opening titles that explicitly references "Five Easy Pieces." This sequence was not included in the finished film. I do not know if it was ever filmed. A compelling aspect of this sequence is the repeated references to Beethoven in the male characters' middle names. Beethoven is not, however, explicitly referenced in the film, or his music even played. But one can imagine that Bobby was intended as a Beethovenesque character. Here is the original opening sequence.
TITLE SEQUENCE:
The Bach-Vivaldi A-Minor Concerto for Four Pianos
PLAYS OVER a series of family album photographs.
Written in careful penmanship beneath each are
names identifying family members:
1ST PHOTO: A man stands in front of the raised
sounding board of a piano, playing the viola.
Seated on the piano bench, accompanying him, is a
woman in a maternity dress: "Isabelle and Nicholas"
2ND PHOTO: A boy of 11, wearing conductor's tails
and holding a raised baton in his right hand as if
about to gesture a downbeat:
"Herbert Kreutzer Dupea"
3RD PHOTO: Another boy of approximately 9, in the
act of playing the violin:
"Carl Fidelio Dupea"
4TH PHOTO: The two boys are now poised behind the
piano. Seated on its bench is a girl of 6, her
hands resting on the keyboard. Written beneath:
"Elizabeth Partita Dupea"
5TH PHOTO: The above family group, seated on the
porch of the Dupea home. All eyes but Isabelle's
are faced toward the camera. She beams upon a 3
year-old asleep in her arms, his head resting
against her bosom. His figure is encircled by the
pen's marking and preceding his name is the
configuration of a small heart:
"Robert Eroica Dupea"
INT. MUSIC ROOM - DUPEA HOME - DAY
BACH-VIVALDI OVER: A 7-year-old BOBBY sits in a chair, his feet
dangling in absent-minded rhythm to a chamber piece
played by his father, his two brothers and his
sister. CLOSE ON a metronome, marking a slow etude rhythm. The CAMERA MOVES from it to Bobby, on the piano
bench beside his mother. As she patiently
demonstrates the etude for him, he places a thumb
in his mouth and leans against her arm. ON THE METRONOME at an andante rhythm. CARL and
TITA, now in their teens, are seated side by side
on the piano bench, playing four-hands with
dazzling virtuosity. The CAMERA MOVES from them to a framed newspaper
article on the music-room wall. Below a photograph
of a 20-year-old young man are the words: "Herbert
Kreutzer Dupea - Seattle's Youngest Guest
Conductor."
INT. RECITAL HALL GREEN ROOM - NIGHT
BACH-VIVALDI OVER:
Bobby, at 10, wearing a dress suit. His mother
combs his hair with maternal concentration.
CLOSE-UP of a program announcing a Dupea family
recital. The CAMERA SCANS down the bill, over:
Sonata in C Major for Two Violins - Bach - Played
by Nicholas and Carl Dupea. Like As a Lovelorn Turtle - Hendel - Sung by
Isabelle Dupea. Rondo Alla Turca - Mozart - Played by Elizabeth
Dupea. Piano Sonata, Opus 110 - Beethoven - Played by
Herbart Dupea. The CAMERA COMES to rest on:
Five Easy Pieces - Grebner - Played by Robert
Dupea. INT. MORTUARY CHAPEL - DAY
Five Easy Pieces, played haltingly OVER the torsos
of a line of people moving slowly down the chapel
aisle. ANOTHER ANGLE shows a solemn procession of the above, filing by
an open casket holding Isabelle Dupea.
ON THE FAMILY PEW
The CAMERA PANS from NICHOLAS, seated on the aisle,
to the four adult children seated next to him and
COMES TO REST on Bobby. His gaze is cast down to
his lap, as he refuses to look at:
The pale profile of his mother's face resting
within the satin folds of the casket lining, and...
... as the last of the "family friends" pay their
respect: Nicholas steps out into the aisle and,
followed by Tita, Carl and HERBERT, moves down
toward the casket. Bobby rises from his seat and makes his way toward
the aisle, where he hesitates briefly, then turns
and walks up the aisle and out the chapel doors. TITLES END
company.