One of the badtls.io tests is to ensure that a TLS connection with DH params that are too small fails. However, recent versions of OpenSSL (obviously) don't support DH params of 512 since it is a security vulnerability.
To allow for testing to take place, a TLS proxy is placed in front of the web server, configured with small DH params. The simplest option for this is a statically-linked build of socat, using OpenSSL 1.0.2.
The ./compile.sh script will build socat from source, either on Linux or Mac.
For most users, the pre-compiled binaries in bin/ should suffice.