Since $DNA$ is a hydrophilic molecule, it cannot pass through cell membranes. In order to force bacteria to take up the plasmid, the bacterial cells must first be made 'competent' to take up $DNA$. This is done by treating them with a specific concentration of a divalent cation, such as calcium, which increases the efficiency with which $DNA$ enters the bacterium through pores in its cell wall. Possibly, calcium chloride causes the $DNA$ to precipitate onto the outside of the cells or it may improve $DNA$ binding.