These should be really easy to spot, particularly when using the pre-filled candidates. You're looking for a single candidate number in a given cell - the term "naked" is used because there are no other candidates in the cell. Just fill these in straight away, and then eliminate the number as a candidate for any other cells in the same row, column or subgrid.
In the example, the number 3 is the only candidate left in the center cell so can be filled in immediately.