Basically it's a flag to indicate the state of a user. Sometimes you need to be able to disable users, or otherwise affect their 'state' without actually deleting them from the table. That's what the state column is intended for if you use such a system.
As a simple example, think of a temporarily banned user on a forum, you don't want to delete them, so you set their state to banned, and only allow users who aren't banned to log in.
Of course there could be more states to indicate other things, such as an account that hasn't yet been validated by way of confirmation email, or requires administrator approval, whatever makes sense in your user ecosystem really. It could be you don't need any at all, in which case you can safely ignore it.