Freshers week is where you have your (usually mandatory) inductions, get your uni access card(s) and IT logins, and formally complete your registration with the uni.
There are also usually "fairs" for societies running so you can walk around and visit stands for each society and see if you wish to join any. There are also tend to be a lot of drinking and clubbing events on in the evenings.
Your lectures will then start in the following week.