LaTeX does spacing nicely most of the time. But, sometimes it becomes necessary to insert a little bit more of horizontal spacing. This might be between words of text, between math formulas or even between figures placed horizontally beside each other.
The following 3 TeX horizontal spacing macros can be used for this purpose:
\enskip: Inserts 0.5 em of horizontal space.
\quad: Inserts 1.0 em of horizontal space.
\qquad: Inserts 2.0 em of horizontal space.
Inside TeX, the above are defined as:
Note that em is a horizontal measurement unit and its value is dependent on the font being used. That is, it is a measurement unit that ships with a font. It is typically the width of the letter M of the font.
Reference: Pages 60 and 352 of The TeXBook by Donald E. Knuth