The keygrabber works very similarly to bbkeys, which is an excellent tool but had some limitations (and was license incompatible), but with a totally new syntax for the config file, and a couple of new features which makes Fluxbox even more powerful.
For one, the new keygrabber supports chains of key sequences (like emacs)... so you could for example have: Mod1 + M + Mod1 + F to switch to the next workspace (not that anyone would actually use that particular key sequence.)
And if you've typed in a part of the sequence but decide not to continue (abort) then you can hit another key sequence (that you configured in your keys file) to simply abort (with: AbortChain).
In addition to this you can also bind a key sequence to switch between grouped/tabbed windows in a group (with: NextTab and PrevTab).
Finally, for your convenience we (or rather vlaad and tarzeau) have supplied you with two scripts (they both do the same thing) to convert a bbkeys conf-file to a Fluxbox keys-file. (usage of the scripts is described in them). Download: convertkeys or convertkeys2.
Example 5-1. A Fluxbox keysfile
Mod1 Tab :NextWindow Mod1 F1 :Workspace 1 Mod1 F2 :Workspace 2 Mod1 F3 :Workspace 3 Mod1 F4 :Workspace 4 Control n Mod1 n :NextTab
So, as you can see, First there is a modifier, then a key (then a modifier and key again if you wish to have a longer sequence) and finally a colon with an action.
For a list of valid keys actions read a little further down in this document.
And why this is good for you? Well, now you can master your xmms, for example:
If you have some additional keys (e.g multimedia keys), you can control xmms this way if you have configured those keys properly in XFree86:
Example 5-3. Multimedia keys for XMMS
None XF86AudioPlay :ExecCommand xmms -u None XF86AudioStop :ExecCommand xmms -s
I would say check xmms --help for more info, but you are doing this right now probably...