A filibuster is when a member or members of a legislative body debates continuously to delay a vote on a matter. Under normal parliamentary rules a motion to “call the question” stops debate and puts the motion (the question) to a vote. To stop such debate in the U.S. Senate is called cloture and requires a super-majority. It used to require a 2/3rds majority (67 votes) when they actually had a filibuster rule, requiring members to be debating to prevent a vote without passing a cloture motion. In any event, the key is that in order to delay a vote, the person or persons had to continuously be on the floor actually speaking. Because of this, filibusters were not utilized unless the Senator(s) were very strongly against the measure under consideration. Most importantly, the filibuster did not prevent a vote, it delayed it. So, it was not inherently undemocratic.
Perhaps the most famous modern filibuster was Sen. Strom Thurmond’s 1957 record-length filibuster against ending segregation.
For various reasons the U.S. Senate decided to dispense with the requirement to actually be debating, to not allowing a vote unless 60 Senators voted to do so (see, Wikipedia Article). This changed a rule allowing a filibuster to one that allowed a minority to prevent a vote. It kind of worked when there was a willingness to compromise, but has become nothing more than an invitation to subvert the will of the majority by a minority. This is blatantly undemocratic.
Allowing a minority to subvert the will of the majority is inherently undemocratic, but if the Senate wants to preserve its filibuster tradition it should go back to requiring those preventing a vote to stand up in the well of the Senate and debate for all to see their obstruction. However, this creates problems with the Senate doing the work of the people and my view is it is time to end the filibuster rule. I used to be ambivalent about it because I felt it fostered legislation that had broad support. However, it is now used as a bludgeon for pure political purposes and the country is in too dangerous a place to allow a minority to create gridlock.
PS, I was a U.S. Senate Page in the summer of 1969.