TeTeX is an implementation of TeX for Linux or UNIX systems.
TeX takes a text file and a set of formatting commands as input and creates a typesetter-independent .dvi (DeVice Independent) file as output.
Usually, TeX is used in conjunction with a higher level formatting package like LaTeX or PlainTeX, since TeX by itself is not very user-friendly.
If you are not a TeX expert, you will probably also want to install the tetex-doc package, which contains documentation for the TeX text formatting system.