Gridlock, in politics, refers to a situation in which very little in the way of laws can get passed by a government.
In the United States, gridlock is most likely to occur in periods of divided government. The current situation in the US is a perfect example of this. The House of Representatives is strongly controlled by Republicans while the Senate is controlled by Democrats. The President is a Democrat. As a result, measures that are passed by one house of Congress are quite likely to be rejected by the other house. This is certainly the case with House measures such as the one repealing the health care program.
Gridlock can also happen even in periods when government is not divided. One major reason for this is the fact that small groups of Senators have so much power to hold up legislation.
Our system was designed by the Framers to promote gridlock. They feared a powerful government and so they set up a mulit-part government in which many different parts have to agree in order for anything to get passed. This is an intentionally inefficient system that lends itself to gridlock.