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.