While it's not an official medical term, "freshers' flu" typically refers to a variety of colds and illnesses that students are susceptible to as they socialise with new people at the start of term. Despite what the name suggests, you don't have to be a fresher - a student in your first year of university - or even a student at all to catch freshers' flu.
Germs spread by coughs and sneezes are the main ways in which colds and flu are passed on. To avoid catching freshers' flu you should practice good hygiene. This includes washing your hands often with soap and warm water, using tissues to trap germs when you cough or sneeze and binning used tissues as quickly as possible.
It's also important to stick to a healthy diet and get plenty of vitamin C. Public Health England also advises students to ensure they're up to date with all their vaccinations including MMR and MenACWY.