When working the first row of crochet, a pattern may state that you work in the back “bump” loop rather than the top loops of the chain.
Chain the number in the pattern then flip the chain over to see the back bumps created in the chain.
Your hook goes through the bump loop then yarn over and pull through. Continue all stitches stated in the pattern in these back “bump” loops.
When you’re done with the first row, your edge has a lovely clean look. Building your next few rows, you’ll be happy it’s neat and even if you’re adding a border, it’s easier to work along the edge to build the border.
I think it’s neater, tighter and especially if you don’t add a border, makes it look very professionally trimmed.