Reroll allows to change the type of the gem. For example, if you need one more Luck gem to Super Merge, but you have a lot of Activity gems, you can use this feature to receive a Luck gem instead.
To use the reroll you will need two gems of the same type and level. To start the reroll:
Open the 'Inventory' tab,
Tap on a Gem you would like to reroll,
Choose the 'Reroll' option in the opened menu,
In the following screen, press the '+' button to add a second gem of the same level and type,
Check the reroll price at the bottom of the screen and press the 'Reroll' button.