It means to turn something around. For example:
Er wendete. (He turned the car around.)
Er wendet den Pfannekuchen in der Pfanne. (He turns the pancake in the frying pan around)
Wende bitte das Laken. (Please turn around the bed-sheet).
Bitte wenden! (Please turn aroun (as in a letter at the bottom of the page).
In these cases, it is quite commonly used. In others, you can use it, but it sounds pretty oldfashioned and mostly replaced by "umdrehen"
Wende dich nicht um! = Dreh dich nicht um! (Don't turn around)
sich an jemanden wenden: to turn to somebody
Du kannst dich immer an mich wenden. (you can always "turn to me"...come to me)