Reactive means exactly that - they react to certain triggers and contexts in a way that puts themselves or others at risk or at least, involves behaviour you'd rather they didn't practice.
Some dogs are reactive on the lead - because it traps them and reduces their options from flight/fight/freeze/fiddle about to just 'make a bloody big racket'. Those dogs might well not be remotely reactive off lead but might also have a shit recall.
Some dogs are of course reactive on lead and off it, aggressive to other dogs, or children, or squirrels etc etc.
Some dogs are reactive but not aggressive, just lacking in manners and with a rude approach to other dogs and a poor recall.
Some dogs reactivity might be to run away, dangerously across roads, in a blind panic.
One of mine can be a bit reactive on lead because he's excited and silly and makes ridiculous hound noises and thinks people wish to cuddle him, but he's also 30kg of hairy dribbly hound, who has no recall because once his nose is down his ears are switched off - he genuinely has no clue we've been shouting him, and similarly, just expects us to be right behind him as he beetles off after a scent and is utterly bewildered to find we are not (but this hasn't given him any ability to switch the ears on when scenting).
So he stays on a long line and we avoid getting near people who might look at him or engage with him (he's fine if people ignore him, but not if it looks like they'd like to say hi!) as his excited 'I want to cuddle that person' noises sound bizarre (Strangled howling, the boy cannot hold a tune).
Not an aggressive bone in his body, he wouldn't know how. But would still be a flaming liability off the lead.
So if he got off the lead he would say hi to other dogs, try to solicit cuddles from people (sit on their feet then when they bend down to pat him, or fall over him, squash them with a full GBGV cuddle), slime them with dribble... and after a couple of minutes of that he'd be nose down and halfway to the next county, oblivious to everything but the scent.
Reactivity does not determine what the dog is like off the lead, and aggressive behaviour is only one way a dog can be a liability to themselves and others.