Theseus represents one of Shakespeare's more rationally minded characters in A Midsummer Night's Dream. Shakespeare uses Theseus to juxtapose reality and rationalism with illusions and irrationality, thereby portraying his central theme of reality vs. illusion. This passage in particular portrays Theseus's rational side.
When Theseus declares that "[l]overs and madmen have such seething brains" what he is saying is that both lovers and lunatics are equally emotionally intense. The word "seething" can be translated to mean "boiling," or "frenzied," which are other ways of describing intense emotionalism. By saying that both lovers and lunatics are emotionally intense, he is equating lovers with lunatics. He is arguing that lovers are just as emotionally driven and irrational as lunatics, thereby showing us that both lovers and madmen embrace illusion rather than reality.
Theseus further states that both lovers and the insane embrace illusion rather than reality by adding that both have such vivid imaginations that believe far more than rationalism would ever allow. In other words, both lovers and madmen believe things beyond reality; they believe in illusions rather than reality. When Theseus says that both "lovers and madmen" have "[s]uch shaping fantasies," he is referring to having imaginations. The term "apprehend" can be translated as "conceive," believe, or know, showing us that he is declaring that lovers and lunatics imagine things beyond reality.
Therefore, ultimately, this passage can be interpreted as showing us that Theseus is comparing lovers to lunatics and saying that both equally embrace illusion rather than reality.