Pleats (or plaits, as they used to be known) are folds formed in clothing by doubling the cloth back on itself to create a fold. Pleats are commonly found on trousers, as well as kilts and skirts for women.
Pleats on pants have become extremely unfashionable in recent times (especially on khakis), as they have come to be associated with boring middle-management types, and tend to billow up in unflattering ways when you sit down.

Avoid this look
Pleats do have the advantage of helping to make your legs look longer with their strong, vertical lines, so they can possibly work on thinner guys and taller guys with long legs, but most guys should avoid anything with pleats.

Pleats vs no pleats