What it is
Replays the handshake the way a panel of real clients would perform it, covering Chrome, Firefox, Safari, Edge, OpenSSL, Java, iOS and Android across a spread of versions. For each, it reports the protocol and cipher that would be agreed, or that the client cannot connect at all.
Why it matters
Tightening a TLS configuration always locks somebody out, and this shows who. Worth running before you drop an old protocol version, and again afterwards to confirm you dropped only what you meant to.