The Assign block is used to assign a value to a variable. This assignment is absolute, which means the new value will erase the previous one.
Here is an example : in my story, I start with an amount of Y gold coins. But I will unfortunately lose all my gold coins and I get back a purse of 10 gold coins.
To recreate this event, I add a block Assign whose variable is Y and I set its value at 10 by double clicking in the block. The mathematical operation is now Y=10.
The assigned value will correspond to the nature of the variable. So a boolean variable can only have a true or false value, a numerical variable, a numerical expression...