Mustard Mine can perform a large number of tasks for you, the viewer; these commands are all created on a per-channel basis, but some will be common across channels, perhaps with some small adjustment to wording.

• !calc provides a very simple in-chat calculator. Enter a mathematical expression and get back a result: !calc 1 + 2 will tell you that the answer is 3. This can work with decimal fractions (!calc 3.5 + 7.5 will respond with 11) but not with symbolic values (sorry, no !calc pi here).
• !followage reports how long you’ve been following this channel. Flex that longevity!
• !addquote will add a channel quote, great for remembering funny or adorable moments. Start with a user name !addquote MustardMine I am a bot to cite the person; include emotes as desired, and they will be retained. This command is usually restricted to moderators, with the corresponding !quote command (to view quotes) being available to everyone.
• !shoutout someperson (equivalently !so someperson) to recommend a broadcaster to your community, with a fancy Twitch shoutout as well as an in-chat description.
• !hypetrain reports in chat about the current Hype Train status.

If the command you seek is not mentioned here, it is possibly a channel-specific command. The broadcaster may have made some information public on the corresponding page, eg /channels/rosuav/commands for Rosuav’s commands.