Sound
What it really means
As slang, "sound" means "fine, okay, no problem", or describes a person who is decent and trustworthy: "he's sound". Common in the north and among young people.
Context, tips & common mistakes
Sound, especially in the North and Midlands, is quiet high praise for a person: 'he's sound' means trustworthy, decent, alright-in-all-weathers — understatement doing the work superlatives do elsewhere. There is no higher compliment hiding behind it; that is the compliment.
As a standalone reply — 'Pint later?' 'Sound.' — it just means 'fine, agreed', the northern cousin of 'cool'. You can safely adopt this one early: it fits texts, pubs and workplaces, offends nobody, and using it correctly makes you sound naturalised faster than almost any other single word. In the South you will hear 'sound' less; 'sorted' or 'safe' occupy similar ground regionally.