Decriminalisation means relaxing the penalties so you get a civil fine, not a criminal charge. Jaywalking may get you a ticket, but it doesn't make you a criminal. Many people advocate for the decriminalisation of many drugs, including hard drugs; the idea is that even if we don't think heroine use is okay, locking someone up for using heroine isn't solving the problem.

Legalisation means making it actually legal, as many US states have now made marijuana, or as NZ has made sex work. It can still be regulated or taxed, but in general its fine.