Grains typically refer to the weight of the bullet.
For instance, 9mm bullets come in everything from 95-147gr.. Most 9mm ammo is either 115gr or 124gr bullets. Some people prefer a heavier/slower bullet like a 147gr hollow point, while other prefer a lighter faster bullet.
It goes back to the age old arguement of momentum over energy... 45ACP vs 40S&W, vs 9mm...
In reality, don't get caught up in bullet weight, unless you are reloading for a specific competition, and a specific gun. For defensive shooting, and quality, name-brand hollow-point will usually do.
Some prefer a heavier bullet for shooting through heavier clothing in winter, as a heavier bullet will often penetrate deeper, in testing... In reality??? There aren't enough willing test subjects out there, looking to get shot for the sake of science.
Hope this helps