In simple words, ssh-audit is a tool for ssh server and client auditing. For example, you can use this tool:

Scan for OpenSSH server and client config for security issues
Make sure the correct and recommended algorithm is used by your Linux and Unix boxes
Check for OpenSSH banners and recognize device or software and operating system
Lookup for ssh key exchange, host-keys, encryption, and message authentication code algorithms
Alert developers and sysadmin about config issues, weak/legacy algorithms, and features used by SSH
Historical information from OpenSSH, Dropbear SSH, and libssh
Policy scans to ensure adherence to a hardened/standard configuration