Manual

Most UNIX systems have online manual pages. The types of manpages have sections which they are grouped in. Here is the layout of BSD:

section 1 general commands (tools and utilities) section 2 system calls and error numbers section 3 the C libraries section 4 special files and hardware support section 5 file formats section 6 the games section 7 miscellaneous information pages section 8 system maintenance and operation commands section 9 kernel internals

When someone says to you "man 6 tetris" he means that you should read the tetris manpage in section 6 of the manpages. Sometimes the same name manpage exist but in different sections, one example is the fstat manpage it exists in sections 1 and 2. To see the manpage for fstat in section 2 you would type

$ man 2 fstat