Custom events are a mechanism by which you can track things that happen throughout a visitors' session in your form.
You can find out how to set up the tracking of custom events here, in our docs. You will almost certainly need developer resource in order to be able to get this set up.
The primary (but not only) use case for this functionality is to track specific error messages.
For example, each time an error is shown to a visitor, you would push that as a custom event into Zuko, then be able to examine what error messages were shown, and how often:
The chart above shows the total count for each event, along with a breakdown of those occurrences in session that ended in abandonment, and completion.
You can also view a tabular breakdown of this same information below:
The table provides you additional information - specifically what % of sessions the event happened in, and an average number of times it occured for each person that saw it.