ServerSignature is HTTP header information that is sent by a (the) web server. This information includes server version, operating system and the installed modules on the server, hackers might exploit this information for vulnerabilities. Here is how ServerSignature can be disabled

In technical words mod rewrite is a way to direct server to respond in a specific way to request it receives. In words understandable for general public mod rewrite allows creating user and search engine friendly links

One of many things that we discuss with our clients is rebuilding their websites to reflect the new image of their company. Normally, these design and content changes are accompanied by a domain name change. Obviously, no one wants to lose the traffic that comes from the search engine high rankings and to start doing the SEO work from scratch neglecting the effort that was invested in ranking the original website.