In every turn, you try and play one or more cards from your hand onto the table. You can only legally end your turn if all cards on the table are grouped in valid melds. There are two types of meld: runs and sets.
A run is a meld of 3 or more cards of the same suit (or color), and ascending values, like 456 or JQK . The Ace (A or 1) is played low or high, according to the acting Aces rule.
A set is a meld of 3 or 4 cards of the same value, and different suits or colors, like 444 . A set cannot contain more than four cards, and cannot contain two cards of the same suit or color.
Adding cards and manipulation
Instead of playing a new meld from your hand, you can also add your cards to melds that are already on the table.
You can even take away cards from existing melds to combine them with other cards, as long as you make sure that at the end of your turn, all cards on the table are part of a valid meld.
It is not allowed to take cards from the table back onto your hand.