High Sierra (macOS 13.3) has had four updates, now sitting at 10.13.4. It usually takes a new macOS about six months, or four updates, to shake out most of the bugs.
This creates a problem for Mac users that want to stay up-to-date because Apple brings out a new macOS every year. That is why so many Apple techs, including me, suggest avoiding upgrading to a brand new macOS. For years, these have been introduced in the Fall, and are generally safe to use by the following summer, with the latest updates to them.
To complicate matters, every Mac comes with the App Store and a notification system that insists we stay current with all Apple software. If you buy a new Mac, it usually comes with the latest operating system. With an older Mac, you can update your macOS, or you can upgrade to the latest macOS.
Let's say you bought your Mac a couple years ago and it came with macOS El Capitan. At some point you were prompted to update from 10.11 to 10.11.1, and to each successive update until 10.11.6. After that you started seeing a larger graphic suggesting you upgrade to macOS Sierra (and later to High Sierra).
An upgrade installs an entirely new operating system on your Mac. Updates make relatively minor changes to your existing OS. Updates usually don't create problems. They tend to solve them. Upgrades introduce a world of changes that can be disruptive, even if everything goes well. But there are times when upgrades do not go well at all, and High Sierra upgrades can stall or leave your Mac unusable. The solution can be as dire as erasing and reinstalling your data from a backup.
The vast majority of upgrades to High Sierra will go smoothly. But there is a risk to upgrading your operating system and the risk seems to be especially high with this macOS. Before you upgrade, consider why you are upgrading. Is it because you "want to stay current", or is there a feature you need, or a problem that you need to solve? There should be a good reason to upgrade.
If you have decided to upgrade, be sure you have an up-to-date backup, or preferably two backups. Ideally you want a local Time Machine backup and a secure cloud backup like Backblaze. I also advise checking for malware before an upgrade. I use Malwarebytes and DetectX.
There can be good reasons to upgrade a Mac. For a balance between security and stability, I prefer macOS Sierra (10.12). Apple doesn't offer to upgrade your Mac to Sierra in the App Store, but you can contact me if you need help with that.