If you are making a cake, for example, and the recipe calls for 1 cup of mini-chocolate chips, you may see something like "Add 3/4 of the mini-chips to the batter. Reserve the remaining 1/4 cup for the frosting."
You keep food stores "in reserve" in case you are expecting a bad storm.
But to simply ask you mom to not let your brother eat your share, it's "save some for me, please."
You'd sound oddly formal if you said "Mother, kindly keep a portion of dinner in reserve for me pending my late arrival home." You'd be correct, but absurd.