The amount of power you have when you’re negotiating something is proportional to how good your best other option is if this negotiation fails.

That ‘best other option’ is sometimes called your ‘Best Alternative to a Negotiated Agreement’ or ‘BATNA’.

If you’re negotiating a job offer of $X, but you have another offer from another company of 1.5*$X, you can negotiate pretty hard. Whereas if this is your only offer and the alternative is you can’t afford to eat… you might have to just accept the offer.

More accurately you could argue that it’s partly also based on what the other party thinks your BATNA is—it’s higher risk but if you can make them think that you have solid alternative options, they may be inclined to give more concession.