Instead of drawing two cards at the start of your turn, you may, under certain conditions, choose to pick up the discard pile instead. This is called "picking the pack".
Picking the pack means picking up the entire discard pile. This can be a boon or a burden, depending on what all has been discarded, so it's important to pay attention to its contents as the turns pass.
In order to pick the pack, you must be able to play the top card immediately. This means you may pick the pack if:
It also follows that you may not pick the pack if the top card is a 3.
If you have not yet opened, you may still pick the pack, as long as you are able to meet your opening threshold.
You may only use the top card from the pack in your opening. However, once you have demonstrated your ability to validly open, you may continue playing cards from the pack during your same opening turn.
Players freeze the pack by discarding a wild card. Lay the first wild card in a given pack horizontally when discarding to indicate this as play continues.
When the pack is frozen, a new rule for picking it is added: Even if you are adding to a partial Canasta on your table, you must have at least two of the card you are picking in your hand and play these immediately along with the top card.
If you are still uncertain, see this handy interactive flowchart.