GOP : Group Of Pictures, is one of principle of MPEG.

1) I, B P frames :
The video stream compressed by MPEG, is coming with informations on transmitted pictures. It send many types of encoded pictures, this is I, B, P frames.
The I frame is the reference picture. It's the fisrt of a GOP. The MPEG decoder is waiting for this I frame before starting decoding. It handles the maximum of information of the picture and doesn't need other datas.
The B and P frames are differential pictures, they take I frames as reference and are using less datas and bitrate.

2) The GOP length :
GOP is the group of frames starting by the I one.
For example : IBBPBBPBBPBB.

The GOP length (checked in this website) is the number of B and P frames between 2 I frames.
For a broadcaster, the goal is to have the minimum bitrate to send a TV channel. So he could choose to use the maximum of B,P frames between I ones to decrease bitrate. But, by doing this, the timing used before the first picture to be displayed when a TV channel is chosen, is becoming very long. A gop of 25 will take 1 seconde to display first picture of the TV. At this time, we need to add the time for the receiver to tune the frequency to be received and to decrypt the sream if it is not free to air.

3) Values used :
In DVD the GOP is usually between 12 and 15.
In DTT MPEG2, the GOP is between 22 and 25.
In MPEG4, Gop are more unstable, because it's not picture used in I,B,P frames, but slices (part of picture). The GOP can take values from 20 to more than 40.