Frank Ford is a rich citizen of Windsor, married to one of the "merry wives" (that would be Mistress Ford).
The most important thing to know about Ford is that he runs around the play doing a bad impersonation of Othello. What do we mean by that? Well, like Othello, Master Ford is totally cray-cray and thinks his wife is cheating on him, even though she's not. But unlike Othello, nobody ever takes Master Ford seriously. In Merry Wives, his jealousy is played for laughs and he winds up becoming an object of ridicule.
After hearing that another guy wants to hook up with his wife, Ford decides to spend 95% of his time trying to catch her cheating. This dude even goes so far as to disguise himself as a guy named "Brooke" so he can spy on his wife and Falstaff. (Gee. Why don't you just ask her if she's cheating, Master Ford?)
The other 5% of his time is spent delivering big, dramatic speeches that would be kind of scary if Ford wasn't so ridiculously funny. Check out the way he goes berserk when his wife tells him he's got no business worrying about "buck washing" (aka laundry that needs to be bleached):
Buck! I would I could wash myself of the buck!
Buck, buck, buck! Ay, buck; I warrant you, buck;
and of the season too, it shall appear. (3.3.130-134)
Yikes! Why is Ford flipping out about the word "buck"? Well, when he hears the word, he automatically thinks of a male deer (aka an animal with horns). And horns are a common symbol for cuckolded husbands (aka guys who get cheated on by their wives). In other words, Ford is overly sensitive about the subject of horns because he thinks he's been made into a cuckold.
Okay, so, it's true that Mistress Ford deceives her husband and lets him think she's been unfaithful but, that's not why Ford is so distrustful. As Mistress Ford reminds us, the guy has a long history of mistrusting his wife. When she gets a steamy (and unwanted) letter from Falstaff, she says "O, / that my husband saw this letter! it would give / eternal food to his jealousy" (2.1.86-88). Obviously, Ford has had some issues for a long period of time.
So, what's this guy's problem? Well, when it comes down to it, Ford doesn't trust any women because he thinks they're all "frail" (were born with a weakness of moral character). Check out the way he criticizes his pal for trusting his wife:
Though Page be a secure fool, an stands so firmly
on his wife's frailty, yet I cannot put off my
opinion so easily: (2.1.202-204)
Okay. Get your highlighters out because here's something you need to know: in Elizabethan England, a lot of guys thought that all women were born flawed and that their "frailty" made them prone to cheating on their husbands. That's why the idea pops up in so many of Shakespeare's plays, like Hamlet declares "Frailty, thy name is woman!" (Hamlet, 1.2).
So, it's not just that Master Ford is super jealous. He has a problem with women in general. Our evidence? When he hears that his maid's aunt, "the old woman of Brentford," is visiting, he goes nuts and accuses her of being a witch:
A witch, a quean, an old cozening quean! Have I not
forbid her my house? [...]
Come down, you witch,
you hag, you; come down, I say! (4.2.151-158)
And after all these insults, he proceeds to beat the "old woman" after he screams a bunch of insults at her. Of course, "the old woman" isn't actually a woman at all. It's Falstaff, who's been tricked into dressing up as an old lady, which is the only reason why this is remotely funny.
But, here's the thing: Ford doesn't know that he's beating up a man disguised as a woman. He thinks he's pounding on an old lady. Most theater companies play this for laughs but, we've got to admit that it kind of freaks us out.