Many metadata fields are available. However, only a few are seen by the Kindle, which doesn't really use most of them.
Probably the most relevant are author, title, language (apparently this is useful if you load up extra dictionaries), enabling text-to-speech, disabling the clipping limit.
Unfortunately, you can't really set the metadata properly via HTML in the source.
But if you're converting your own documents, you can try using an ePub-style .opf. You can set author, title, date, publisher, etc. very easily just by adding the relevant lines and both Calibre and Mobipocket Creator will respect and add those values to the finished file.
There's a
useful tutorial here with an example OPF you can cut and paste and fill in. Ignore the section about adding metadata to the HTML header via Webuilder. It's redundant to the OPF and I have no idea whether it actually works for what you want.
For editing the metadata on pre-created Mobi files, if you're on Windows, you might like to try the
Mobi2Mobi GUI tools, created by fellow MR members. If you're a Linux or Mac user, you can also use the command-line script versions, but they're a bit trickier to install and get the hang of.
Hope this helps and welcome to MobileRead!