A tinderbox is system designed to test builds and report failure. In the FreeBSD case, tinderboxes build world (the base system), GENERIC, and if applicable LINT kernels.
Tinderbox failure reports are sent to development lists to notify developers of breakage. If you're receiving these reports it's likely you're subscribed to such a list. Notable examples of such lists include: -alpha, -current, -ia64, -sparc64.
This happens when no one has fixed the initial breakage. The idea is to annoy someone, hopefully the person that broke the build, into fixing the build.
Dag-Erling Smorgrav currently runs all the tinderboxes by cross-building from i386.