These should be quite easy to spot, particularly when using the pre-filled candidates. It applies to rows, columns and subgrids but is easier to explain by discussing one of those at a time.
You're looking for a pair of candidate numbers used in two different cells within the same row - the term "naked" is used because there are no other candidates in the cell. As this pair of cells must contain both occurences of the paired digits (but we don't know which way round just yet), we can eliminate any other uses of either digit in the row.
This technique is also known as locked candidates and is essentially a variation of the pointing and claiming techniques, but with two digits at once.
N.B. You should only use this strategy once all candidates in the row (or column or subgrid) have been filled with candidates.
In the example, the numbers 7 and 9 occur together as a pair in the row so we can eliminate any other uses of either 7 or 9 in the same row. This in turn then exposes 6 as a naked single in the first cell of the row.