An IP number is an Internet address. It consists of four numbers from 0 to 255 separated by decimals, with several special addresses. For instance "127.0.0.1" always means "this Internet address, originating this". So each machine refers to itself by this address, regardless of how others refer to it. There are also broadcast addresses and gateway addresses.
IP numbers are allocated in C blocks of over 64,000 addresses and D blocks of 256. All numbers which can be routed over the Internet are presently allocated so one must buy addresses one seeks to use, or rent them from the actual owner.
To block IP is to prevent communication to a computer from such a number or range of numbers.