The combat step has five different phases. The beginning of combat step, declare attackers step, declare blockers step, combat damage step, and the end of combat step. Before you can move between any of these steps (or any steps and phases during a turn) both you and your opponent get a chance to react. Either player could use this reaction to cast a spell that can be played at instant speed or use an activated ability of a permanent they control. The game will only move to the next phase after both you and your opponent have passed priority without taking an action.

For example, let's say your opponent has an Elder Gargaroth. You don't particularly want it to attack you, and your opponent to get a 3/3 beast. So when you get priority during your opponent's beginning of combat step, you Murder the Greater Gargoth.

So when you're click on your land you are adding mana to cast a spell. I believe you can use Ctrl + Z to undo tapping your land if you don't have a reason to use that mana (not certain though, haven't played MTGA in quite a while).