The word shot has several meanings:

1. A single attempt at doing something. When a basketball player takes a shot at the basket, they're trying to score. When you take a shot at solving a difficult puzzle, you're giving it your best effort. People might say “give it a shot” when they want to encourage you to try something new, even if you're not sure you'll succeed.
2. A photograph. A photographer might get a great shot of a sunset or ask everyone to smile for a group shot. The word comes from the idea of cameras “shooting” pictures, capturing a moment in an instant, like an arrow hitting its target.
3. An injection of medicine with a needle. You might get a flu shot before winter or need a shot when you visit the doctor. Though nobody enjoys getting shots, they protect you from dangerous diseases.
4. The act of firing a weapon, or the sound it makes. In old Western movies, someone might say “I heard a shot ring out.”

When someone is described as calling the shots, it means they're the one making the decisions and being in charge.