Dissimulation: to conceal one's true motives, thoughts, etc., by some pretense.
Simmulation: the act or process of pretending; feigning. (Random House Dictionary)
Bacon says that dissimulation is a choice when those strong of heart and mind know when to tell the truth and when to withhold it. He speaks of this in reference to politics and the "dissemblers" who withhold truth for less noble reasons and more readily thus making a strong contrast between dissemblers and dissimulators.
He says there are three degrees and the second one applies to average people while the first one, secrecy, applies to those bound to keep others secrets, like clergy and lawyers. The person using the wisdom of dissimulation of the second kind "veils" and protects themselves from prying people. The third is 'simulation" which is not noble and is the attempt to defraud people of the truth in unscrupulous ways.