A drop latch is a pivoting bar (the latch arm) on the gate that drops into a metal hook or slot on the post. Gravity holds it closed. To open, you lift the arm out of the slot. Drop latches are dirt cheap, dead simple, and have been in service since wrought-iron gates were common in the 1800s.
They're not pool-compliant because they don't self-latch reliably (a gentle close doesn't generate enough lift to drop the arm cleanly into the slot every time). They also offer no security against being lifted from outside.
Drop latches still earn their keep on garden, courtyard, and rural gates where security and pool compliance aren't issues.