A method is a named block of code that performs a specific task. Instead of writing the same logic over and over, you write it once inside a method and call that method whenever you need it.
Every method in Java has parts: an access modifier (like public), a return type, a name, and a parameter list in parentheses. You've already been using one method this whole time: public static void main(String[] args). That is a method named main that returns nothing (void) and takes an array of strings as input.
Methods let you break a big problem into smaller, testable pieces. Without them, a -line program becomes impossible to debug because every line depends on every other line.