For the layman, a "WAF" or Web Application Firewall is a service you can place in front of your website to protect it from the many threats out on the internet. It can head off known exploits, mitigate DDOS attacks, and block known bad actors. These tools come in a variety of shapes and sizes and pricing will vary. This is also distinct from a "reverse proxy cache" like varnish. Which can improve performance of a site dramatically, and help with some of these things I mentioned, but in general is kind of dumb, and can't react to threats.