Welcome to the forum.
An All Line Rover is a ‘walk on’ ticket you can buy on the day of travel. As it name suggests it is valid to travel on all National Rail trains in Britain. Note there are some morning time restrictions on boarding or alighting Avanti, Cross Country, EMR and LNER trains before 10am on weekdays at London, Birmingham, Watford, Bedford and Stevenage.
Ordinarily seat reservations aren’t required to travel on trains. During these covid times reservations are compulsory to travel with LNER and highly recommended for many other train operators. Any station ticket office can sort out reservations for you if you tell them which trains you want to travel on.
Note that if you want to travel on a sleeper train you will need a reservation and will need to pay the berth reservation fee if you want a berth.
I’d recommend purchasing a day or so ahead of travel as some ticket clerks might struggle to find the ticket on their system (to be fair they don’t sell many).
An ALR is also expensive so I suggest you put together a plan to make sure you get full value from it. Depending on your plans you might find it better to buy some regional rover tickets along with some Advance tickets.