I have off peak return ticket from London to Didcot Parkway. Screenshot is attached. I am not sure what timings are allowed on this ticket. I checked this question, according to which travel restriction can by accessed by code FP. However, I am getting confused about what are the train timings which are allowed in this ticket. Can someone help? Also what does 'optional reservation' mean? I can travel at other timings also right?
The information provided by The Trainline is admittedly very poorly explained, and appallingly formatted. The information provided by National Rail is a bit better.
Each restriction code states when and/or on what train(s) the ticket is not valid and the days and/or dates that these restrictions apply. Restrictions do not generally apply at weekends or on bank holidays unless specifically stated
Since you have a two character code, it is a restriction code, which describes when you can't travel. If you put 'FP', which is your restriction code in, you get a list of times when your ticket would be invalid if travelling from a particular station. Presumably, to go to Didcot Parkway, you will be travelling from Paddington. Therefore, your ticket is valid as long as you don't take a train leaving Paddington between 04:30 and 09:19.
Just as a note, there is no reason to book through the website thetrainline.com. They charge a booking fee. If you use the National Rail website you will be able to buy tickets without paying a booking fee.
When you buy train tickets in the UK, there are many ways in which to buy them - you can buy tickets that are restricted to a single journey, tickets which are restricted to a single day, tickets which are only valid for the non-commuting period of the day (off peak) etc etc etc.
Train tickets in the UK sometimes come with required seat reservations, but those are usually the type of ticket which is restricted to a single specific journey.
What you have there are actually two things:
A ticket which is valid for off peak travel - the National Rail page that they link to for restriction code FP shows the times you are NOT allowed to travel with that ticket, as these are considered "peak" times.
A seat reservation for the specific journey you bought the ticket for. This does not actually mean you have to travel on this specific train, as the ticket is more open than that - so long as you do not travel during the restricted times, you can take any train on that route, you just won't have that seat reserved. A seat reservation is not required for this ticket.
When you book a ticket online in the UK, it will almost always give you an itinerary. This is just how the online booking process works — you search for a journey first, then you book a ticket that is valid on that journey. Your ticket is guaranteed to be valid on that itinerary, but since you've bought a flexible ticket, you're not required to travel on it. You have seat reservations on the 19:15 train, but if circumstances require it you can travel on any train allowed by the restriction code FP (as you correctly identified in your question); you will just have to find an unreserved seat to sit in as your seat will obviously not be reserved for you on the other train (alternatively you can change your seat reservation at a ticket office up to the day before travel free of charge). Seat reservations are not compulsory on ordinary UK trains (except for sleeper trains and Eurostar), so you have the option of just boarding and trying to find a seat.
As for restriction code FP, the page you linked is indeed quite confusing. I think the page is trying to list the times between which the ticket is NOT valid, when boarding at various stations. For instance, it's NOT valid for trains timed to leave London Paddington between 04:30 and 09:19. It's quite strange not to have any text explaining this though!