Short answer:
The numbers *before* the slash are the IP address of a network interface.   The number *after* the slash is the bitwise subnet mask (netmask).   A netmask indicates how much of the IP address is network and how much is host.   All devices on a given routed network must be configured with the same netmask for proper operation.

Long answer:
Many TCP/IP addressing tutorials are on the web.   Googling on the terms "tcpip tutorial address bitwise" turns up several that look to be credible.   My personal favorite is at PerlMonks - Don't Use Regular Expressions To Parse IP Addresses!, of all places.   Not that I'm biased, yo.