This is a common subtrope of Geo Effects, where boggy or swampy areas drastically impede your movement. So long as you remain in here, you'll move at a snail's pace, vulnerable to passing enemies. Your jumping, if applicable, may or may not be affected by this, and in fact you may be able to move quickly by repeatedly jumping in and out of the bog. In some cases the bog may actually be hazardous to your health, dealing damage over time, acting like quicksand and swallowing you whole if you spend too long within, or applying some unpleasant Standard Status Effects.