It's the corollary to 'Know your rifle". You need to know your ammunition. First in general terms. What is the bullet weight? What is the purpose of that bullet? How far does it go and retain enough energy to be effective? How much does it drop between various distances?
Then specifically, in your rifle, how does one type of ammunition from one company perform? If you are working on accuracy for target shooting, the differences can be surprising. I have a .22 bolt action rifle that I bought for the CMP Sporter matches. Trying to decide what ammo to use, I bought a box of every type of .22 long rifle on the shelf in my local gun shop.
This gave me an excuse to go to the range. Targets at 50 yards. The rifle has a scope, so my targets consisted of a black dot paster in the center of a paper plate. All the ammo performed fine, but one stood out. In that rifle, from a supported rest, Wolf Gold Medal Match was the clear favorite. 5 round groups all in the paster, usually 3 to 4 of them touching.
I could do no better, so my search for ammunition for that rifle was done. Now I could practice, knowing that the rifle and the ammo were not the cause of the scattered pattern I was seeing when I shot offhand.
This leads to a decision for every serious shooter. When do I stop trying to buy ammo and start reloading my own? Not .22 of course, the design and price of that ammo does not lend itself to reloading. But all the centerfire cartridges. Because if you really want to know your ammo and how it performs in your rifle, reloading isn't optional. And that will open the door to an ongoing series of posts.