These numbers are in GHz and usually refer to the 2 wifi frequency bands available. They are not bandwidths, which indicates how much data can be carried per unit time.
Wifi on 2.4 GHz is the most common band. There are only 3 channels that do not overlap (there are 11 channels, but only 3 do not overlap). Since it is the most common and only 3 non-overlapping channels, it is very crowded and has lots of potential interference and degradation. It is also close to other devices that use 2.4 GHz. It has better wall penetrating ability than 5 GHz.
5 GHz is much less crowded and supports the higher 802.11ac standard, that gives a higher bandwidth. There is also more channels. There are also less devices that use 5 GHz. With that, it is less crowded frequency, less interference and less degradation. You will get more consistent speeds. However, it won't travel as far as 2.4 GHz, and does not penetrate walls as well.