Cindy, in the US Senate, there is no time limit on how long Senators are allowed to speak when addressing the floor (ie speaking in front of the other Senators). A bill cannot be voted on until debate has finished and every Senator who wishes can have their say on the floor. If a Senator doesn't like a bill that will probably get enough votes to pass, he can delay a vote by taking the floor and not stop speaking. This is a filibuster.
However, 60 Senators (out of 100 total) can end all debate by voting for cloture. Since the Democrats control the Presidency and both houses of Congress, the only avenue the Republicans have to prevent Democratic bills from becoming laws is the filibuster.
With Spector becoming a Democrats, the party now has 60 votes in the Senate and could shut the Republicans out of debate and pass bills much easier.
In reality, the 60 vote threshold doesn't mean much since Senators can vote freely regardless of what their parties position is.
It does mean that the Republican party is becoming smaller and weaker. 200,000 Pennsylvanian voters left the Republican party in 2008. Spector saw the writing on the wall there.