The basic strategy chart shows you the best action for every situation. There are different charts for different rule variations, especially the number of decks being used and whether double after split (DAS) is allowed.
What is meant by the best action? It’s the action that has the highest expected value (EV), so you get the best chance of winning the most money. But for a lot of situations, even the best EV is still negative, so you’re just minimizing your losses.
If you count cards, you’ll know when to go against the basic strategy for some situations.