web.xml is Deployment Descriptor which consist all the basic configuration servers( like Tomcat) need to know at the time of deployment like context path, url mapping, listener, filters, error and security.
you can mentioned following in web.xml -
- What Servlet you are going to use. What URL you map with them.
- Listener - classes when any event going to happens.
- Logger , error-page your welcome files.
- Other configurations files path.
You can go through the below link for more details-