Explain what assertions are and how they are used in programming.

Assertions are special constructs in programming that check the truth of a certain condition during program execution. If the condition is false, an exception is thrown (for example, AssertionError in Java), which helps identify errors and incorrect states in the code.

In Java, assertions are used with the keyword assert:

int value = 5;
assert value > 0 : "Value must be positive";

Here, it checks that value is greater than zero. If not, the program will throw an error with the message "Value must be positive".

Assertions are typically used for debugging and verifying developer assumptions, and can be disabled in production builds to improve performance.