A superscript is a number or letter that's written slightly above another character. When you write out the mathematical expression "x squared," you write the 2 as a superscript, smaller than and raised above the x, like so: x2.
Superscripts are frequently found in mathematical formulas and as footnotes in academic texts. You'll see a superscript printed smaller and higher than the rest of the text, which makes it stand out—so it's clear what it indicates, as in the case of exponents or footnotes. Superscript comes from the Latin superscriptus, "written above," and the roots super, "above," and scribere, "to write."