One's subcutaneous "spare tank" of insulin, which has yet to be used by the body.
Because no insulin injection is immediately 100% absorbed by the body, the yet to be used insulin stays under the skin, the system drawing on this "reserve" as needed. Any such insulin effects that last after the insulin's expected action is over are also known as carryover in some literature.
The larger your insulin dose, the larger the insulin depot in your subcutaneous fat tissue. Having a large vs smaller insulin depot means the effect of the insulin can be less consistent--more chance of it varying from day to day. This same insulin depot is the reason why it can take anywhere from 2-5 days to see any effect of insulin dosage changes.
This handy "extra tank" is also there to give you a hand if you miss or are late with an injection.
On the other hand, when you do miss an injection, your "spare tank" is very low by next injection time, and so the next injection may have less effect than expected!
This free downlodable chapter of Dr. Hanas' book contains an easy to understand explanation of an insulin depot on pages 10 & 11.