Some people have used these words interchangeably, but -
An energizer is something you put with the yeast up front to get it started off strong and healthy. Rather than just picthing the yeast on top of the must, I always make a yeast starter, which includes an energizer.
A yeast nutrient is something you put in the fermentor, typically after a few days of fermentation. It supplies vitamins, minerals, and such to help the yeast stay healthy. With some yeasts, if they don't get extra nutrients, you can get the rotten egg smell or a stuck fermentation. I have seen some instructions where the nutrient is added up front, just like the energizer.
For most, but not all grape wine kits, if you use the yeast that is supplied, no extra nutrients are required - they put the nutrients in with the juice. Mosti Mondiale kits sometimes require a (supplied) nutrient to be added per instructions.
If you use a different yeast than what was supplied with the kit, you often will have to add a nutrient, so check with the yeast supplier.
For fresh/frozen grapes and most every non-grape wine, adding yeast nutrients is required.