From the product page:

mitm-proxy is an Java-based SSL proxy that acts as a "man in the middle". In other words, proxied HTTPS requests are terminated by the proxy and resent to the remote webserver. The server certificates presented to the client (i.e. a web browser) are dynamically generated/signed by the proxy and contain most of the same fields as the original webserver certificate. The subject DN, serial number, validity dates, and extensions are preserved. However, the issuer DN is now set to the name of the proxy's self-signed certificate and the public/private keys of the proxy are used in creating the forged certificate. These forged certificates are cached (in memory) by the proxy, for better performance.