A user error is anything the user does that is ‘wrong’. That could include clicking on a link that doesn’t get them any closer to their goal, it could be failing to see/read relevant information on the page they are on, it could be typing into the wrong field (eg typing a search query into a log-in box), it could be not completing all required fields on a form or completing some fields incorrectly.
One thing to be careful of is that different people have different process tracks - if there is more than one possible route to the end goal, don’t “mark down” people who choose a different route to the one you would have chosen. What is most important is that people get to their goal, not necessarily how they get there (although any route that involves a lot of backtracking and undoing should be considered as user errors).
Maybe instead of watching what they do wrong, see what they do to complete the task and make that route easier?
I was told once that there are 2 main types of users (I’m know there’s more), those that are happy to take click on big graphics to drill down to the content and others who want a menu to drill down immediately to the destination. Point is, there’s lots of way users interact with a website, you can’t give them just one route.