$\ce{Co·6H2O}$ does not exist. But, if it would exist, il would be a special sort of metallic cobalt, surrounded by 6 water molecules fixed around each $\ce{Co}$ atom. This substance would be electrically neutral. It would exist in a sample containing no other substance.

$\ce{[Co(H2O)6[^{2+}}$ does exist. It is a charged species (cation) that exists, but does not exist alone. It must be accompanied by one or several negative ion, like $\ce{SO4^{2-}}$ or $\ce{2 Cl-}$. There must be the same number of positive charges and of negative charges in any sample containing this ion. So there must be as many $\ce{[Co(H2O)6]^{2+}}$ cations than say $\ce{SO4^{2-}}$ anions. In solution, these ions are independent and can react differently. But the number of positive charges must always be equal to the number of negative charges.