As noted by ThatT1Feeling it's a shorthand way of referring to a certain cabin config.

BA has a bewildering selection of 777s, if you go to: Product you will see some further information.

So it looks like your flight will be served by a 14F/48J/40W/124Y configuration(*), and the Heathrow based variants of that configuration have the newer Thales IFE rather than the ancient Rockwell Collins stuff. So while it is unlikely the 777 will be topping anyone's favourite aircraft any time soon, hopefully your flight should be somewhat more pleasant at least in terms of the cabin.



*meaning 14 First (F), 48 Business (J), 40 Economy Plus (W) and 124 Economy (Y)