remaining amount; leftover portion
Refers to the part of something that remains after some has been used, taken, or consumed. Often used for food, money, time, or tasks.
Please put the leftovers in the fridge.
We bought new equipment with the remaining budget.
余り is a broader term for a surplus or remainder (including mathematical remainders), whereas 残った分 specifically refers to the leftover portion after something has been used or consumed.
Phrase composed of the past tense of 残る (to remain) and 分 (portion, part).