This maintenance release fixes a severe bug in 4.9.3, which will cause sites that support automatic background updates to fail to update automatically, and will require action from you (or your host) for it to be updated to 4.9.4.
Firstly, it is to the credit of the WordPress Community that such things are found so quickly and fixed, but it does suggest that the testing regime is more like ants. They look organised, but actually the individuals work at random and ‘fall over’ the solution collectively.
Serious issues is that this last update broke the auto update facility. So, if your site used to update automatically. It will now be stuck on 4.9.3 and requires intervention. This is a big issue and you must log into your WordPress admin and ask for the update.