An open return ticket is where you don't have to specify what day or time you will be using the return portion.
There are several different types of open return ticket available:-
Anytime Return - the outward ticket of an Anytime Return is valid for five days, including the date on the ticket. You must return within one calendar month of the outward travel date specified. For example, if you bought an Anytime Return for the 1st of March you could wait until the 5th of March to make your outward journey.
Anytime Day Return - you must travel on the date specified on the ticket but you can travel at any time peak or off peak
Off-Peak Return - your outward journey must be on Off-Peak trains on the date specified, and you must return on Off-Peak trains within one calendar month
Off-Peak Day Return - your outbound and return journey can be on any Off-Peak train on the date specified.
Super Off-Peak Return - you can return on any Super Off-Peak train within one calendar month.
If you’re unsure about when you can travel, check the train operators website