Private property is basically the stuff you, and only you, own but do not participate in using yourself though you get whatever benefit that stuff contributes to. For example, a factory is the private property of a factory owner, though the factory owner (boss) doesn't actually work within the factory and instead hires people (wage laborers) to work in the factory producing stuff, though the boss collects whatever profit the workers in the factory produce.
It is also know as "impersonal property" and is considered the defining part of capitalism
(most notably P.J. Proudhon
) and communists contrast private property with personal property, personal property being things like the house you live in, your bed, your guitar, your computer, your shoes, etc. which you are actively using for yourself and thus have the right to exclude others. Private property on the other hand creates an illegitimate authority and, according to them, must be abolished.
My boss operates an apple orchard on acres and acres of private property he inherited from his parents and hires us to harvest all the apples for him, and while he makes a great deal of money off the apples he pays those of us who actually pick the fruit $6 an hour, but he's able to because he owns the orchard.