Per day means a class feature, item function, or other use per day ability can be used up to a maximum number of times each day. For example, Barbarian rage 5/day means that barbarian rage can be used 5 times each day. Spellcasters are also limited in how many spells they can cast per day.
Characters regain all per day uses when they rest.
DnD 3.5 comparison
This usually means between midnight of one day and the midnight of the next, though it could also mean a maximum usage within a total 24 hour period, or that each usage becomes usable again once 24 hours has past for each individual usage.
For example, if a character has druid wild shape 2/day and uses both wild shapes he must wait until 24 hours since the first usage has past before he can wild shape again. If 24hrs has past since the first wild shape use but not 24hrs since the 2nd attempt the druid would only have regained a single usage.
- Per day works the same way as it did in NWN.