There are of course many themes in MacBeth. One of them is similar to Hamlet; how much stock should one place in information from the supernatural world? The play was written with King James in mind. James had a keen interest in all things supernatural (he had also written a book on witchcraft) and most scholars agree that these subjects would have been of interest to him. Queen Elizabeth, not so much.
Macbeth has just met the three witches, and like Hamlet, is questioning the veracity of his encounter with the supernatural. Is the information from the witches good or bad? Is information from the spirit world good or bad? I think now just as then, that is still a relevant question. He is a strong and steadfast man (except around his wife who manipulates him like a puppet...a theme that would have resonated with audiences of the time.) Yet, the witches have scared him...it's messed up his hairdoo and made his strong (seated) heart shake and tremble. Also, note that in English we have the expression "The heart is the seat of the emotions." Seated here means still, quiet, firm, relaxed. He's saying that his heart (or center of emotions as it was seen then, and still is today), which is normally resolute, is now afraid.
I think it's important to see that MacBeth has some similarities to Hamlet, but unlike Hamlet, he is an emotional doer; Hamlet is a cool thinker. The kind of internal thinking that MacBeth does is different than Hamlet. Hamlet sees ghosts and agonizes over what to do. Macbeth doesn't agonize over what to do. He thinks about things, realizes he is scared, but knows that as a man he's gotta do something, then makes up his mind and lives with the consequences. I mention all of this because as far as I can recall, Shakespeare did not use similar imagery to talk about Hamlet, so the line is saying something specific about MacBeth's personality--that he is more emotionally driven than Hamlet.