Rashi means that although they obstensively went to pasture their father's flock, the Torah hints to us that their main intention was to feed themselves.
The Terumas Hadeshen in his sefer באורי מהרא"י explains as follows:
We can explain that the logic behind this teaching is that
although some
words are dotted to teach us that there is hidden meaning,
nevertheless, the hidden meaning of the word never completely leaves
the simple
meaning, because the word is certainly written. Therefore, in this
posuk
the hidden meaning is derived from the fact that the root of the word את
is
אות
which forms the words אותו (him/it) and אותם
(them).
And we find that these words sometimes mean 'himself' or
'themselves', like in the posuk concerning the nazir in
Bamidbar(6,13) “This is the law of the nazir: on the day that the days
of his being a nazir are completed he shall bring him (יביא
אתו) to the entrance of the
Tent of Meeting” the phrase יביא אתו means
"he shall bring himself", as Rashi
explains. And he adds that this is one of the three places in the Torah
where R. Yishmael teaches that the word את
and its suffix are
understood to be reflexive, the other two places being in Vayikra
(22,16) “and so cause themselves (אותם) to
bear iniquity and guilt”, and in
Devorim (34,6) “and he (Moshe) buried himself (אתו)
in
the valley”.
Therefore, here also the hidden meaning is that they pastured
themselves.
The full English text can be found here