It means to ask God to forgive your sins and for you to forgive those that sin against you.
It is a central tenet of the Christian faith.
“...means to ask God to forgive your sins and for you to forgive...”
Not quite.
A bit subtle, but most important: we ask G_d to forgive our offenses EXACTLY AS we forgive those who offend us.
I used to tell my Christian High School students it’s the most dangerous prayer you can say.