What does it mean to decriminalise drugs?

Some countries have legalised cannabis, but it has not been introduced for hard drugs in any country.
As Ireland prepares for a Citizens' Assembly on drugs use that is likely to kick off before summer, there is significant confusion about what drug policy reform might mean, especially around the difference between decriminalisation and legalisation.
The one thing there is broad agreement on is that our current drug policy is failing. Ireland has among the highest rates of overdose in the European Union. The EU average for drug-related deaths is 16.7 per million of population, while in Ireland, there are 73 deaths per million of population. There are more than 3,500 people waiting for a detox bed, of whom 72% are male and the average age is 32.