The Rolling type of User retention report shows the percent (or the number of users) who came back in a specific day or any day after specific.
The report supports Day 1, Day 2, Day 3, Day 4, Day 5, Day 6, Day 7, Day 14, Day 30, Day 60 and Day 90 retention. If the player returns to the game, he will be counted in the appropriated column and all columns before.
For example, the player joined the game on 01'July. Then he played the game on 03'July and on 09'July.
Rolling retention for this case shows the data for Day 1, Day 2, Day 3, ... Day 7 retention.