The number of auctions you can win is limited to a maximum of 13 within a 28-day period. These are broken into four different lock types, based on the item you win. For example, if you bid and win six items within the Green Lock category, you are no longer eligible to win any more auctions from that category for 28 days from the date of your first win. (For more info on this, see Auction Limits)

To eliminate these locks, you can bid and win a Lock-breaker auction that allows members to reset their limits on one or more lock categories. Lock-breakers are available in a variety of colors and combinations including one for each lock type. Once you win a Lock-breaker auction, it opens up that lock or category of locks. The 28-day limit will only begin once you win your next auction in that category. 

For a more detailed description, go to Help -> Auctions -> Lock-breakers.
To see these auctions, go to Categories -> Special -> Lock-breakers.