Generally speaking, you're going to be protected by the Fourth Amendment if the police come and knock on your door. They can knock, but they can't search your home unless they have a search warrant. They need to have the warrant and let you see it before coming in.
However, following are two key exceptions that it's wise to be aware of:
The police will probably start by asking you if they can come inside or if they can conduct the search. You don't have to let them, but some people say yes just because they're intimidated by the uniforms or don't realize they can say no. Once consent is given, the police can come inside.
Even if you tell them not to search your house, letting them in may allow them to see something that is "in plain view." For example, if they come in and see what they believe to be illegal drugs, they then may be able to look into it further because they didn't have to "search" to find it.
2. Threat of destruction
Another big exception is if they think you have evidence in the house and you're going to destroy it immediately. They can then sometimes conduct a search without a warrant if getting one would take too long and allow the destruction to happen.
For example, they may think you have important videos, pictures or text messages on your phone or computer that need to be recovered before you delete them or destroy the device.
If police conducted a search of your home, knowing if that search was legal is crucial, as evidence from an illegal search in Maryland can be thrown out. Your Maryland attorney can provide guidance.
Source: Electronic Frontier Foundation, "Know Your Rights," Hanni Fakhoury and Nadia Kayyali, accessed March 04, 2016