What “Dem” means
Dem means they and them. Placed after a noun, it also marks the plural: "di pickney dem" means the children. Before a noun it can mean "those".
Examples
Dem a get married
They are getting married.
Di dog dem a bark
The dogs are barking.
Gi wi di key dem
Give us the keys.
How to use it
Everyday grammar used by everyone. Plural dem is optional when the plural is already clear from a number or a word like nuff.