Look at the bottom right 3x3 box. See that there are two squares that have 567 and one that has 75.
Think about it. If one of the squares is a 5 then the others must be a 6 or 7. If one is a 6 then the others must be a 5 or 7. If one is a 7 then the others must be a 5 or six. Since each 3x3 box can only have each number once, you can eliminate all of the other 567 possibilities in that 3x3 box. The center of that 3x3 box must be a 3. You can keep going from there.
This technique works on 3x3 boxes and rows