In the US “sleet” is the term for a winter precipitation that occurs when snow falls through a layer of warm air and melts into water droplets, then re-freezes into ice pellets as it passes through colder air closer to the ground. In many other areas that were part of the British empire that precipitation is called “ice pellets” and “sleet” instead refers to a mix of snow and rain. In the US that’s called a “wintry mix.”
Sleet is basically crunchy snow. Very slightly larger, a bit harder, not really a danger to much of anything it falls on. You don’t get golf ball sized sleet, you get like, half-a-pea-sized sleet.