What does Intel mean in this phrase (collect all intel).

It's short for "intelligence". The phrase means 'collect all information'.

It's a clipped word (back - formation) which means 'intelligence'. In computer language, clipping is a major way of forming new words. Though 'inteligence' is not new, by clipping it gets a new meaning, suchlike 'information' or data. So, 'intel' is 'information' / 'data'.