When we are talking about United States history, writs of assistance are essentially search warrants that did not have to name a specific place to be searched and did not ever expire. These were documents that allowed British government officials to search wherever they wanted, whenever they wanted to look for smuggled goods. The British use of writs of assistance against colonial smugglers helped bring about the American Revolution. After the Revolution, the memory of the writs led to the creation of the Fourth Amendment to the Constitution.
The Fourth Amendment says that warrants have to specify what place will be searched and what the law enforcement officials will be looking for. The officials have to convince judges that they have a good reason to think that their search will find the things they are looking for. This is very different from how things were with writs of assistance.
Writs of assistance authorized officials to search any place where they suspected that contraband might be. They did not have to get any specific warrants. The writs of assistance served as permanent warrants that allowed them to search any place they wanted at any time. American colonists were upset with this sort of broad governmental power. This anger helped bring about the Revolution.