A choil is a small unsharpened notch or cutout in the blade just ahead of the handle, where the cutting edge stops. It gives the sharpening bevel somewhere clean to run out, and depending on its size it can also give your index finger a place to sit.
The original purpose is manufacturing. A sharpening choil is a small relief cut at the heel of the edge so a stone can reach the very end of the bevel without slamming into the ricasso. Without it, the last quarter inch of edge never gets properly sharpened and eventually forms a step.
The second job is grip. A larger, rounded cutout, called a finger choil, lets you choke up so your index finger sits forward of the handle and right behind the edge. That shortens your effective blade and gives you far more control for detail work.
Listings rarely distinguish the two, so read the photo. A notch you could not fit a fingertip into is a sharpening choil. A scalloped cutout an inch across is a finger choil.
A deep choil costs you cutting edge. On a three-inch folder, a finger choil can take half an inch out of the working edge, which matters if you slice a lot of long material. On a bushcraft or carving knife the control is usually worth it.