Apple's WebKit browser engine is responsible for rendering browser content such as HTML and CSS.
Safari Web Content is a software library that displays web content using Apple's WebKit browser engine. It is mostly utilized by Apple's
browser and the iOS browser and operates as a separate process on your Mac.
If something goes wrong with WebKit or Safari Web Content, the browser will be unable to perform its functions and may stop, restart, or crash (as is the case when this pop-up appears).
Check for update
The simplest — potential solution is to make sure your Mac and browser are both up to date. Install any updates to macOS and the newest version of Safari. After your computer has rebooted, try running Safari again.