In its generic form, as pictured here, the wheel represents the cycle of life, enlightenment and divine power. A wheel might also represent a wheelright.

Specific types of wheel symbols that might be found in the cemetery include the eight-spoked Buddhist wheel of righteousness, and the circular eight-spoked wheel of the Church of World Messianity, with alternating fat and thin spokes.
Or, as with all cemetery symbols, it could just be a pretty decoration.