Validity range.
A slot interval during which a transaction is considered valid. Outside the window, the chain rejects it.
A "best between" date stamped on the transaction. Outside the window, the network throws it out.
In depth
Every Cardano transaction can carry a lower and upper bound on slots, `invalidBefore` and `invalidHereafter`. The transaction can only be included in a block whose slot is inside that range. Plutus scripts see this range in the script context as a POSIX-time interval (not the "current" slot, there isn't one inside a script) and use it to enforce time-based rules: vesting windows, auction deadlines, expiry timeouts.
Example
A vesting validator checks that the lower bound of the transaction's validity interval is at or after `unlockTime` (POSIX time). Since the ledger only accepts the transaction inside that interval, the unlock time has certainly passed.