See this website for a great step-by-step explanation.
In brief:An indulgence is “the remission before God of the temporal punishment due for sins already forgiven as far as their guilt is concerned.”
This means that when we sin, we need repent, ask God for forgiveness (to "revoke" the eternal punishment for them) and we also need to there is also a need to remit the temporal punishment. This means that we need to do what we can to correct the effects of our sins or make things right.
For example:
If you hurt your brother, he might forgive you, but he still has to heal from the effects of what you did (the pain takes time to go away, the bruise might need to fade, etc.)
If you make a mess because you are mad at your mom, she might forgive you, but you still need to clean up your room.
If you steal your sister's favorite toy, she may forgive you, but you still need to return the stolen toy.
Poor souls in Purgatory are still suffering the effects of sin in their souls as they get "cleaned up" before they can go to Heaven. These people have already been forgiven by God and will eventually get to Heaven, but it will take some time for them to "clean up their rooms" (a.k.a. make up for the effects of the sin on their soul).
There is something WE can do while still on Earth to help them!
We can offer something called a plenary indulgence for them so that all the “clean up” needed in Purgatory for their sins on earth is completed. It's like when you decide to help someone clean up a mess you didn't help make, just because it's the right thing to do.
Offering a plenary indulgence is just one way Catholics can help each other.
The Saints in Heaven (the Church Triumphant) can pray for us here on earth while we (the Church Militant) can pray for the souls in Purgatory (the Church Suffering). (see image below)
Life with young kids can sometimes feel like running around putting out one fire after another. So how can we start teaching our babies and toddlers about Jesus?