A naked pair occurs on a sudoku board when two cells in a row, column, or box have exactly two candidate numbers left. When this happens, those two numbers can be eliminated from the other cells in the same row, column or box.
When you make pencil marks in your puzzle, naked pairs will be very obvious (which is why they are called “naked”). If you see only two numbers in one cell and the exact same two numbers in one of that cell’s siblings, you have found a naked pair. When it happens, any occurrences of those two numbers can be eliminated from the other siblings.
As you can see, finding naked pairs is a useful technique for making progress on a sudoku puzzle. Since this technique does not immediately reveal the number for a cell, it is considered an advanced technique. Advanced techniques usually only help you cross out candidates which will eventually lead to revealing a number.