This is a mail filtering language that is supported by some email clients or extensions.
In theory it lets you write complex filtering rues using a GUI within your mail client, and not require that you login to your panel account (or mailboxes.yourdomain.com) and create limited rules with a basic web GUI.
I haven't actually used a provider that support Sieve, so I can't say how well it works in practice. If it is rarely supported or the tools for it have fallen out of favor and aren't being maintained, then it may not be worth implementing. But the extension I linked to above seems to still be getting updated to work with the latest Thunderbird releases.