When two numbers can only go in the same two squares of a row, column or box, those squares hold them. Every other option in the two squares goes.
A naked pair shows itself: two squares with the same two marks. A hidden pair is buried among other marks and shows only when you count the places for each number.
For a row, column or box, count the places for each missing number. Two numbers with the same two places are a hidden pair.
Once the extras are gone the two squares are a naked pair. They share a box here too, so the rest of that box can't be 4 or 9.
Boards that need it, with the owl's hints when you're stuck.