Active dry yeast consists of dehydrated yeast granules and some of the yeast particles are dead due to the drying process. The yeast needs to be proofed, which basically means, woken up and checked that it is alive , before you can proceed with the recipe. This is done by placing it in warm liquid, with a little sugar to feed it. It should begin to go frothy in a few minutes if it is alive and ready to use. If this doesn't happen the start again with new yeast. The process used to make instant yeast doesn't kill any of the particles and it is also a much finer powder. This means that it can be mixed with the dry ingredients and will dissolve as soon as the liquid comes into contact with it. This type of yeast is mostly used in bread machines.