Jigjag, you will find these abbreviations frequently on this forum and elsewhere.
“BRB” is “Big Red Book”, ie Chambers (which is doubly daft because even fossils like me don’t have a book but use the Chambers apps).
“LOI” is “Last One In”, ie the clue which most puzzled me and/or I could only solve when I had all the crossers (or had a final shaft of illumination).
You may also come across “my COD”, “my Clue Of the Day” ie my favourite clue because of some outstanding wordplay or quirky definition or whatever.
TTFN