This technique is used to eliminate pencil numbers. Use pencil numbers to mark the possible numbers for each cell.

A naked triple is where, within a row, column or jigsaw piece, there are three cells where the only possibilities for all are the same three numbers. 

As each of these cells must be filled with one of these numbers, they cannot appear anywhere else within the same area, so the pencil numbers for these can be removed from any other cells in the same area.

This can occur where not all numbers are present in all squares. 

For example, in the following example, the numbers 4, 5 and 6 are the only options for three cells within a jigsaw piece.

While we don't yet know which is which, each of these cells must contain one of these three numbers. As a result, none of the other cells within this jigsaw piece can contain these numbers, so they can be eliminated:

This is the result: