I was about to post the same question myself but decided to search first. I think its something like points earned towards Heaven or something.
I don’t worry about them too much. I don’t care if something gives me an indulgence, I do it because I love God and love doing what He asks of me. Thinking of how many indulgences one gets distracts me from the knowledge and love gained from the prayer or act.
Hope to get some more knowledgeable answers.
Okies - think of it this way. When we sin our sins have spiritual consequences in that venial sins damage our relationship with God and our spiritual health, mortal sin destroys it almost completely.
Our sins also have temporal (earthly) consequences. Some are obvious - if we lie to someone we hurt them and also hurt ourselves by losing their trust. Others aren’t so obvious.
All of these are penalties that have to be paid too. If you lose someone’s trust you have to put in the work and effort to regain it, if you steal you have to try to make good what you’ve stolen.
But what happens if you don’t or can’t do so while alive? Imagine Hitler sincerely repenting and making a good confession half an hour before he dies. Justice demands that he pay SOME price for his evil, but the fact that he repented means that, as God’s mercy demands and in accordance with Christ’s promises, he must be guaranteed heaven eventually.
This is where the concept of purgatory comes in. That any temporal punishment due to our sins must be paid off before we can enter heaven. It is also a place where we are purified of those minor tendencies to sin that we all have, since nothing imperfect can enter Heaven.
The idea of an indulgence is that part or all of the punishment you are due to pay in purgatory has been ‘worked off’ while on earth, by the doing of certain acts or the saying of certain prayers.
Of course we should pray out of love for God. The fact is love for God does bring rewards, does it not? Salvation itself for starters. This is another one, of a different sort.
A lot of the indulgences are attached to prayers such as the Rosary or to acts such as spending a certain amount of time reading scripture - in other words things that a lot of us do do anyways. It’s just a further incentive to us (or at least I’ve found it so for me) to persevere in doing them at times when we may be tempted to give up and slack off.
Then again indulgences can be earned by us to benefit the souls in purgatory, and not just ourselves. Now I don’t know about you, but I’m not entirely confident that I’m going straight to heaven when I die

and if I go via purgatory then I sure hope all y’all back here will be praying to help make it that much quicker and easier. So it’s only fair that while I am able I should do the same for others in purgatory too.
Hope this all helps, I know it’s long but indulgences aren’t all that easy a concept to explain.