1.
If you keep back part of something, you do not use or give away all of it, so that you still have some to use at a later time.
Roughly chop the vegetables, and keep back a little to chop finely and serve as a garnish.
= set aside, hold back
PHRASAL VERB : V P n (not pron) , also V n P
2.
If you keep some information back , you do not tell all that you know about something.
Neither of them is telling the whole truth. Invariably, they keep something back.
PHRASAL VERB : V n P , also V P n (not pron)