If the police suddenly arrive at your home, in person or at your vehicle stating that they have a search warrant, it means that they've obtained permission to search a location or person and to seize anything that is evidence of a crime. The search warrant itself will be addressed to you, as the person who is to be searched or whose property is to be searched.
In most cases, law enforcement does need to have a search warrant before searching your property, but that's not always the case. For example, if there is evidence of a crime in plain sight, the officer may obtain it immediately.
What is needed to obtain a search warrant?
The police need to show that there is probable cause that a crime did occur and that the evidence supporting that claim is at a certain location or on a specific person. They need to show that they or others have reliable information about the location of evidence.
Once the police have a search warrant, they may search the areas listed in the warrant. For example, the warrant may state that only your backyard is able to be searched. As such, the police should limit their search to that area. Keep in mind that there are exceptions, like if they have to search further for their own safety or if you allow them to search further by providing permission. It's typically in your best interests to contact your attorney if you have an officer arrive with a search warrant, so you know your rights and can protect yourself.
Source: FindLaw, "Illegal Search and Seizure FAQs," accessed May 22, 2018