Even if you are a very mediocre negotiator, you might be able to easily eg obtain a somewhat higher salary when starting a job versus just accepting a given offer if you're not aware that negotiation is possible/expected.
(This doesn't work in all situations, some employment markets are highly regulated and don't have market rates, negotiation is more likely to produce better results if you are in a stronger negotiating position...)
If you have an hour, read https://www.kalzumeus.com/2012/01/23/salary-negotiation/
If you have a few hours, read through the book "getting to yes"