"Proactive" is the opposite of being "reactive." In business, for example, where you will likely find 99% of the use of "proactive," a company that just responds to the crisis of the day is being "reactive." Customers call, complaints are made, problems are noted. The only action is in response to these complaints or problems.
A "proactive" company tries to anticipate what the problems are going to be and finds solutions before they have crises.
This is all true, but there are still some speakers of Br Eng who raise an eyebrow when they hear 'proactive'; it's a neologism - and it seems to have 'bedded in' more in USA than in UK.