@Struggling1 My original comment wasn’t very helpful.
Here’s the official, technical explanation from the Catechism of the Catholic Church (CCC).
CCC 1471
"The doctrine and practice of indulgences in the Church are closely linked to the effects of the sacrament of Penance.
What is an indulgence?
“An indulgence is a remission before God of the temporal punishment due to sins whose guilt has already been forgiven, which the faithful Christian who is duly disposed gains under certain prescribed conditions through the action of the Church which, as the minister of redemption, dispenses and applies with authority the treasury of the satisfactions of Christ and the saints.”
“An indulgence is partial or plenary according as it removes either part or all of the temporal punishment due to sin.” The faithful can gain indulgences for themselves or apply them to the dead."
So in plain English: an indulgence is a certain pious work (a certain prayer, reading the Bible for a certain amount of time, making a pilgrimmage, etc.) which, when obtained along with the Sacraments and prerequisites of getting it, help to bring you closer to God and remove any potential punishment which you may deserve for sins of your past. Indulgences are not dogma, but they are Roman Catholic doctrine (there is a subtle distinction between the two).
Here’s a parable: let’s say you steal $100 from Mom’s purse. Mom is very mad at you and grounds you for a month when she finds out. A day later, you sincerely apologize to Mom and she accepts the apology, but you’re still grounded. The guilt has been forgiven but the temporal punishment remains for your sin. Your Mom tells you “if you go shovel the deck, you can be ungrounded and watch TV tonight.” So you go shovel the deck and Mom ungrounds you - guess what? Mom offered you an indulgence, and you did the indulgenced work and obtained a plenary (full) indulgence from Mom.
In this parable the theft of Mom’s money is sin, and Mom is God. Grounding for a month is temporal punishment for sin. Your sincere apology to mom is the Sacrament of Confession. Her forgiveness is sacramental Absolution. Mom telling you to go shovel is God offering indulgences through His Church. Going and shoveling is the Indulgence.