2.4GHz and 5GHz are two different Radio Frequency Bands used for wireless networking signals. 2.4GHz was the wireless standard for a long time in the wireless networking world. However many devices also utilise the 2.4GHz band such as but not limited to; cordless telephones, wireless keyboards and mice and even children’s toys.
This means that in some circumstances the 2.4GHz frequency may have a lot of interference in busy areas, 5GHz is now being used by many wireless manufacturers and due to the lack of other devices on this band reduces the chances of wireless interference
However more and more networking devices are supporting both frequencies and both bands have varying advantages that make it difficult to decide on a clear choice and the decision becomes more a question of which frequency your devices support.