Describes that something happens (Per/Each/At time) when put with acounter
一人ずつゆっくりと入り下さい。 Please enter one person at a time, and slowly.
メニューに載ってるもの一つずつちょうだい。 Give me one of each thing on the menu, please!
It can also be used with some set nouns etc.
このお酒は強いので、少しずつ飲んでください。 This alcohol is strong, so please drink a little bit at a time.