\hskip <dimen1> plus <dimen2> minus <dimen3> \vskip <dimen1> plus <dimen2> minus <dimen3> \mskip <dimen1> plus <dimen2> minus <dimen3>
The commands \hskip and \vskip produce horizontal and vertical glue respectively, while \mskip produces horizontal glue in math mode. These commands skip over a distance of <dimen1>, which can be stretched by <dimen2> or shrinked by <dimen3>. Either the plus <dimen2>, the minus <dimen3>, or both can be omitted (the omitted values are taken to be zero). Any of <dimen1>, <dimen2> and <dimen3> is a TeX dimension and can be negative.
Should a line break occur immediately before or after one of these commands, then the intended distance/glue will not be inserted!
Example: the following command produces a horizontal glue of 3cm which can be stretched by 5mm:
\hskip 3cm plus 5mm