The deadline for major league teams to decide whether or not to tender contracts to arbitration-eligible players is tonight. To put that in plain English, players with three to six years of major league service time have their salaries for the year decided by an arbitrator, unless they can reach agreements with their teams first. By tendering a contract to an arbitration-eligible player, a team essentially accepts that the salaries doled out by arbitrators - which are always above the major-league minimum salaries that rookies make, but are usually less than a player might make as a free agent - will be a factor in the player's salary for that season. The arbitrator's decision about the player's salary will be broadly predictable, and will, in almost every case, rise each year until the player becomes a free agent, so there are some cases when a team decides a player isn't worth taking to arbitration and non-tenders him.
↵I'm not sure whether that qualified as plain English or not. (If not, you can check out Pirates Prospects' uber-detailed primer.) But the important things for you as a Pirates fan are as follows.
↵1) The Pirates have five players - Ross Ohlendorf, Lastings Milledge, Joel Hanrahan, Ronny Cedeno and Jeff Karstens - about whom they have to make a decision. Of those, the first four will almost certainly be tendered. I bet Karstens will be too, although that's less certain.
↵2) The Pirates already effectively made decisions about several arbitration-eligible players by designating Zach Duke, Andy LaRoche and Delwyn Young for assignment, essentially getting rid of all three of them.
↵3) If a team does not tender a contract to a player, that player has been non-tendered and is a free agent. The Pirates will be looking through the list of non-tendered players, once that becomes available, to find players they'd like to sign. Chief among those is Twins shortstop J.J. Hardy. The Twins have lost interest in Hardy, and he seems likely to be with another team within the next few months, but it's unclear whether the Twins will non-tender him, or whether they will offer him a contract and then trade him. In either case, the Pirates, who could use Hardy's blend of hitting ability and defense, could pursue him.