The 'berth' of a tent is a guide the manufacturers give to a tent in regards to how many people the tent can sleep.
When choosing a tent it is a good idea to remember that this is just a guide advising you how many people it is possible to sleep, not how many it is practical to sleep. For example, most one man tents are only big enough to fit one person full stop, no allowances are made for 'wiggle room' or storage of gear. The gear you do have with you will, more than likely, have to be left outside of the tent where it will be open to the elements. Some tents feature a small area where gear can be stowed and kept out of the rain between the inner tent and flysheet. The same can be said for all berths of tents so if you would like to comfortably sleep two in a tent, we would always suggest a three man tent and so on. In larger four man and up tents there is normally a lot more living and storage room provided.