To understand MIDI communication, you have to understand a little about bytes and bits. A byte is a packet of data used to store information. In MIDI protocol, each byte is made up of 8 bits; bits can only equal to 0 or 1. A sample byte is given below:

11010111

Each 1 or 0 in this byte is a bit. The leftmost bit is called the most significant bit (or MSB) and the rightmost bit is called the least significant bit (or LSB).

Bytes of the form above are binary numbers because they are expressed using only 1's and 0's. We can convert this number to base ten as well:

11010111 in binary (base 2) = 215 in decimal (base 10)

If you need help converting numbers from binary to decimal or vice versa check out Wolfram Alpha. Type in a binary number followed with "from binary to decimal" to get the decimal equivalent. Wolfram Alpha is also great for converting to and from hexadecimal.

Wikipedia is a good resource for more information about bytes and binary.