Hovering or floating text is that sometimes annoying lettering that appears above (and sometimes in front of) a prim.
Hovering text can be any color, can be broken into multiple lines, and can be solid or transparent. Messages are limited to 254 bytes. The distance at which hover text can be viewed is dependent upon the size of the prim. Bigger equals visibility at a longer distance.
Hovering text is set by a script, and will remain even if the script is removed from a prim. The Scrubber script I shared will remove it, or, if the script is still in the prim, you can remove the text between quotation marks in the llSetText line so it reads "". The official way to remove hover text, however, is with the line that follows: