Definition: to make sure that you do not use or give away all of something, in case you need it later.
(This phrasal verb has more than one meaning)
E.g.1: Because we have so many visitors over Easter, I always keep some eggs back in case someone arrives unexpectedly.
E.g.2: Don’t forget to keep some strawberries back so that you can decorate the cake when it is baked.
This phrasal verb can be separated. (E.g. 1 and 2)