Rotel is canned tomatoes with chiles. There are several varieties; original, extra hot, Italian (?), etc.
It's probably best known for being mixed with melted Velveeta to make the dip that you always seem to have at Superbowl parties, but it is also useful in casseroles and anyplace else that you need tomatoes with a little kick.
You'd find it next to the canned tomatoes at the store. Here is a pic: