To focus on and become expert in one particular area rather than trying to do many things (British spelling of 'specialize').
From Latin 'specialis' (of a particular kind), derived from 'species' (appearance or kind). The word evolved to mean focusing on one particular type.
The word 'specialise' only became common in the 1800s when professions actually started requiring deep expertise—before that, people did whatever they needed to survive, so nobody needed a word for focusing on just one thing.
Complete word intelligence in one call. Free tier — 50 lookups/day.