Welcome to the community @tbeca.
A trading session is simply the time when banks and other businesses in a particular continent are open for business. For example the London session in Nigeria is from 9 am to 5 pm daily and it simply means banks/businesses in London are open during this period. It is important for these banks to be open when you are trading because it is from them that your broker extracts the exchange rates for relevant currency pairs. If you trade after the session is closed (meaning after the banks are closed), your broker will manufacture their own exchange rate and give you which is usually higher which will lead to higher trading fees/spread.