What does Mark 5:2 mean?

Mark 5:2 is a bit of a preview for verses 6–8. In Mark 3:9, Jesus had begun the habit of keeping a boat nearby when preaching on the shore of the Sea of Galilee. This allowed Him to teach without being mobbed by the horde of people seeking healing. We aren't told who owns the boat, but it is presumably the same in which Jesus napped during the storm (Mark 4:36–38) and from which He taught the parable of the sower on the western shore of the Sea (Mark 4:1).

"Unclean" is taken from the Greek root word akathartos which describes something as morally stained or not fit for ceremonial use. "Spirit" is from the Greek root word pneuma, which refers to a singular being which has independent agency but no physical form. When used together, they merely mean a demon. The demon-possessed man met Jesus immediately after running some distance (Mark 5:6). It's possible he saw Jesus before the boat had landed. Again, a demon is compelled to approach Jesus despite the danger it may face (Mark 1:23–24; 3:11).

The word "tombs" comes from the Greek root word mnemeion, from which we get the English word "monument." Specifically, it refers to a sepulcher or a sepulchral monument. Burial practices at that time involved laying the body in a larger cave until nothing remained but the bones. The bones were then placed in a stone box called an ossuary for permanent storage. Some of the larger caves would have been big enough for a person to find shelter.