The \mbox command creates a box just wide enough to hold the text in its argument. The text in the box is in LR mode which prevents it from being broken across lines.
The \makebox command is similar except that it allows you to specify a width which is not necessarily the natural width of the text and also to specify the placement within the box.
An \mbox within math mode does not use the current math font; rather it uses the typeface of the surrounding running text.
The \mbox command is robust.