A snag or pulled loop in the knitted pattern can happen when you accidentally pull on one loop. This tightens up the other loops that are surrounding that loop.

To fix this, you will need to pull the surrounding loops a bit to spread out the tension. You can do so by carefully using a needle to even out the tension and/or by pulling the entire fabric gently in various directions. This very often makes the pulled loop smaller. 

If this is not possible (it's harder to do so on more tightly knitted pieces like the french terry sweat fabric in this video), you will need to use a crochet needle to pull the loop towards the inside of your sweater. 

This hides and protects it for outside influences.