"Mark to market" is one of those confusing terms that's been thrown around discussions of the finance meltdown. It's basically an accounting rule that means that a company has to put value certain assets on its balance sheet (in other words, to "mark") at what the market says their worth.
Steven Pearlstein at the Washington Post has a nice description of the term, why getting rid of it has become an obsession of the anti-regulation crowd, and a sensible proposal: