Merriam-Webster defines "socialize" as "to make social; especially : to fit or train for a social environment." So a child who is socialized is, according to this definition, a child who is trained for a social environment. It's a child who is able to interact with others and form friendships.
It's interesting that public school is considered the hub for this sort of training. Many homeschoolers consider their child's participation in their community to be a legitimate form of socialization.
In fact, some parents have great concerns over the socialization that is occurring in public schools today. There is an alarming number of incidents such as suicide, violence (such as school shootings), and bullying. There are also cases where teachers and students have inappropriate relationships.
Parents may be concerned that their homeschooled child is not getting the time with peers that is considered healthy, though. Some homeschooled children do experience a lack of peer-to-peer play time if there simply aren't others their age around. And often, those playmates that are the same age are off in school all day.