You cannot tackle in flag football. The purpose of flag football is to play a very tough and brutal sport, football, without too much risk of injury. In flag football, a 'tackle' occurs when a flag from a players flag belt is ripped off of them, marking them as down.
The only players that wear flag belts in flag football are positions that can carry the ball, like running backs, wide receivers, and quarterbacks. Offensive and defensive linemen rarely wear flag belts, but they sometimes do in case of a fumble recovery or interception.
If a player loses their flag belt or flag prior to gaining possession of the ball, a tackle is called with 'two-hand touch.' A two-hand touch tackle occurs when a player can get both of their hands on the ball carrier at the same time, and the ball carrier is 'tackled.' A two-hand touch isn't as black and white as pulling off a flag. When you pull off the flag, it is obvious that the player is down because the belt comes off. However, with a two-hand touch, it can be difficult to tell whether or not the contact was actually made.