This is a simple extension of the
Naked Pair idea. The three highlighted cells are all part of column 1,
and collectively, they contain only the numbers 1, 2 and 5. Consequently, these numbers cannot appear elsewhere in
column 1, so a 5 can be removed from C1-R7, and 2 and 5 can be removed from C1-R8. Fortunately, this uncovers another
Naked Pair which sets off a chain of simple solutions in the bottom left 3x3 sub-square.
Naked Triples can also appear in rows and sub-squares.