What: It's a test of how much protein is excreeted in your urine during a 24 hour period
How: You collect all your urine for 24 hours in one or more jugs (talk to your lab or nurse for specifics on how they want this done - generally you collect in a plastic container that fits in the toilet that we called a "hat" that then can be poured into the actual collection jugs). You need to keep the jugs cold (or on ice) during the collection for best results. After collection you then turn in the whole thing (all jugs) to the lab and they do the rest of their magic, whatever exactly the do to measure the protein and other things asked of them (if necessary) by the doctor.
Significance: The amount of protein that is "spilled" by your kidneys in a 24hour period is one of the two critical diagnostic criteria for preeclampsia. If the amount is over 300mg AND your BP is over 140/90 (twice over a period of time) then you have met the criteria for a diagnosis of PE). Generally speaking, the greater the amount you are spilling the bigger the problem you may have. Note, however, that there are other things that can cause protein in your urine (none of them particularly good - kidney damage, infections etc...)
What will they do about it? : Well this depends on how much is found, as well as the results of other tests and lab work that is generally done around the same time. Reactions could be anything from as relatively mild as just keeping an eye on things (repeating the collection every few days/weeks) to as severe as immediate hospitalization - again, this really depends on results from other tests and diagnostic criteria, all alone without ANY other things going on a high 24hr protein number may just result in a referral to a kidney specialist for followup (then again, I assume you are at least pregnant to be posting the question here).
I hope this helps...