The first term, a hold, has to do with simply bringing an item to the senate floor. As a rule, the senate puts in place a set of guidelines as to how business will be conducted about a bill, nominee approval, etc. This is done by what their rules call a unanimous consent agreement. When a senator places a "hold" on an item being considered by the senate he's basically saying, "Look, I am saying up front that I am going to do everything I can to obstruct the deliberation of this
item." It's an indication that they intend to filibuster.
Now... a filibuster is often thought of as a means to defeat a bill, which is what it was intended to be. It's a way for the minority to keep the majority in check. So, if someone decides to filibuster a
bill, it's not really going to matter one whit if they don't have to 40 votes in place to back them up, right? Well, yes and no... To break a filibuster the majority leader, Harry Reid has to invoke cloture. The first step is to file the motion for cloture; once that is done no action can be taken on that bill for two days. Then the vote is made to inoke cloture, which breaks the "hold" and business can proceed on that bill. ...but only after a 30 hour post-vote debate period. Then the motion to proceed on the bill can itself be filibustered, along with each amendment attached to the bill. Each of these require their own 2 days of inactivity and 30 hour post-debate period.
In the end, while a filibuster may not actually defeat a bill it can seriously delay and slow down the action in congress. A bill, even an uncontroversial bill such as extending unemployment benefits or continuing funding for our troops in Iraq, or a nomination as uncontroversial as our embassador to Ethiopa (we still do not have one) could take a week and a half to pass instead of a simple vote on the day the bill was introduced.