Second Wind is a Fighter feature gained at level 1 in the 2024 SRD. You have a limited well of physical and mental stamina that you can draw on.
You have a limited well of physical and mental stamina that you can draw on. As a Bonus Action, you can use it to regain Hit Points equal to 1d10 plus your Fighter level.
You can use this feature twice. You regain one expended use when you finish a Short Rest, and you regain all expended uses when you finish a Long Rest.
When you reach certain Fighter levels, you gain more uses of this feature, as shown in the Second Wind column of the Fighter Features table.