A thick space. So if you are using an 11pt font then the space provided by quad will also be 11pt horizontally of course. There are four environments that put LaTeX in math mode.
LaTeX formats mathematics the way its done in mathematics texts.
Simple symbols class 0 have no spaces around them. LaTeX ignores input spaces in mathematics mode. 518 of quad 5 mu -318 of quad -3 mu space after backslash equivalent of space in normal text qquad. There are four environments that put LaTeX in math mode.