I’m not 100% positive but here is what I think it means.
The letters are the Coach Letters.
Letters in inverted type are first class coaches, those in normal type are standard class coaches.
The cup and bycycle indicate the location of the onboard shop and the cycle storage (Coaches C and A)
The end that is chamfered to look like a driving cab is the front of the train (furthest from the concourse at Euston).
The amount of fill on the coach pictures indicates roughly what proportion of seats in that coach are reserved. If you don’t have a reservation you probably want to aim for a coach with few reservations to maximise your chance of getting a seat.
This is all based on personal experience. I don’t know of any official source.