This Sudoku solving technique is built upon the previous one - "Obvious pairs". But "Obvious triples" is not based on two numbers from the Notes, it's based on three. This is the only difference. To understand better, let's take a look at the example.
Look at the top left block. Its three bottom cells contain notes of 1, 5; 1, 8 & 5, 8. This means that these cells have number 1, 5 & 8 in them but we don't know yet where each number is exactly. What we know though, is that 1, 5 & 8 can't be in other cells of this block.
So, we can remove them from the notes.
That is how "Obvious Triples" technique works while solving Sudoku.