In Mario Kart, a POW block is an item that can be used to shrink all other racers on the track, making them more vulnerable to being hit by items or knocked off the track. This can be a very powerful item, especially in multiplayer races. However, there are a few things you can do to avoid being hit by a POW block.
If the POW block works on you, try your items first to defend yourself. POW block can be defended with Super Star, Mega Mushroom, or Bullet Bill. Or else, you need to keep distance yourself with other racers to avoid the impact of POW block. It is easier to get hit by the POW block when you are near to the racer who uses it.