Using a website should be simple and straightforward. A bit like a quick walk down a clear corridor to the exit.
When your website visitor arrives, you want them to find what they need without having to stop and think "what am I supposed to do next". If this interruption happens then it's likely they'll give up and move on to another site.
Complicated shopping carts/baskets and checkouts are places where poor usability can creep in. You can't figure out how to actually buy the thing, or there are so many hoops to jump through you lose interest.
Navigation can be another area to confuse visitors. Tip: If people are resorting to your site search tool to find things then maybe there's a problem with the navigation. Look in Google Analytics > Behavour > Site Search for clues.
Remember that clear corridor? A site with usability issues is like that same corridor but with cardboard boxes piled all the way along and blocking the way. It's a slow and laborious task to clamber over them all to get to your destination.
Reviewing your own site for usability is possible of course. What's even better is getting a fresh pair of eyes with the experience to know what to look for.