It all depends on the type of ticket you’ve purchased. For example, if you buy an Anytime Day Single or Anytime Day Return, you’ll need to travel on the date shown on the ticket.
For Anytime Return tickets, your first journey needs to be made within five days of the date shown on the ticket, whilst the return journey needs to be made within one calendar month.
An Off-Peak train ticket means you can only travel during Off-Peak hours, whilst a season ticket will only be for the specific route your ticket is valid for.
With an Advance ticket, you can only travel on the exact train and time you’re booked on.