Jokers are wildcards that can be used in place of any other card. But depending on the joker rule of a particular game, there may be restrictions in how you can use a joker that already lies on the table.
You can pick up a joker from the table:
There are no restrictions in using jokers that are already on the table.
Only after you replace it with a matching card.
Before you can use a joker that is on the table, you must free it by replacing it with the card that it stands for.
With the latter rule, there may be even more restrictions:
The replacing card must come from your hand.
In games with this rule, you can only free a joker with a card that came directly from your hand. You cannot use a card that was already on the table.
You cannot remove other cards from a meld with a joker.
In games with this rule, you can only add new cards at the front or the tail of a meld that has a joker in it. You cannot take cards off such a meld, and you cannot add a card in the middle (making it split).
You can reuse a freed joker:
Anywhere you like.
You can use a freed joker in any way you like, including adding it to an existing meld on the table.
Only in a new meld with cards from your hand.
You can use a freed joker only in a new meld, where all the other cards came directly from your hand. You cannot add the joker to another meld, or use cards from the table to form the new meld.
Games with this rule
In Rummi, many games use jokers. Some games, like Easy Rummi, have the most relaxed joker rule; others, like Tile Rummy, have more restrictions. The games Shanghai and Jin Rummy, amongst others, have no jokers at all.
Other Rummy type games often use jokers in one way or another. Rummikub® contains jokers, but different editions of the game contain different rules about how you can use them.
If you play such a game, it is best to agree on the joker rule before you start playing. In Rummi, of course, the joker rule is enforced by the rules of the game that you are playing.