I am the new king of the united states and so all of the land contained within the united states is now mine! Now, I am taking your house unless you pay me to NOT take it.
Sometimes you will hear someone say “property is theft”. This is a statement that can easily be misunderstood because it is makes people think that all property is theft and therefor a person should be against private property, this is wrong and is a misreading of the quote by Proudhon, who lived in a time when property could only be owned by the monarchy.
The first thing that needs to be understood is that there are two types of property law-made and labor-made property. Law-made property is theft, labor-made property is not theft.
Labor made property is what most people think of as property. It is your house, it is your car, it is all of the things you have. You accumulated these things through your labor, you either built or created these things yourself, or more likely you put your labor into whatever you are good at and traded that labor for the labor of someone good at building a house or car, etc, using money as the medium of exchange. Libertarians believe this kind of property is sacrosanct and should be respected and is the foundation for a civilized society. It is natural for people to want to improve their lifestyle through the accumulation of things that make life more comfortable. The ability to have possessions of your own is what motivates some people to invent and innovate in the hopes of becoming wealthy.
Law-made property is what I would consider theft. Law made property is when a government/king/bureaucrat or other despot claims a massive land mass. The king cannot homestead that much land, in almost every circumstance in history the land that has been claimed is already being used by individuals who do have claim to the property as labor-made property. The only legitimate claim to property has to be based on labor, it cannot be based on law or force, if it is based on force than it is stolen land. However, this is the basis for all government land, it is also the basis for property taxes. The claim is that the ultimate owner of all property is the government, therefor you have to pay the masters their tribute.