When you take something that's against the law and make it legal, you decriminalize it. If your state rescinds a law requiring motorcycle helmets, they decriminalize riding without a helmet (which still doesn't mean it's a good idea).
As social norms change, lawmakers and voters may choose to decriminalize activities or behaviors that were once illegal. If the legislature of your states decides that a casino will bring in needed revenue, they might decriminalize gambling. The word decriminalize can also be used when things are still technically against the law, but those laws are no longer enforced: "There's still a leash law, but the city decided to decriminalize off-leash dogs on the beach."