Aunt Maud is a rich, snobbish old lady who likes to meddle in the affairs of the people around her. She takes a special interest in her pretty niece, Kate Croy. You could say that she has great expectations for Kate.
The #1 expectation she has is that Kate will marry a rich or famous man. This is the turn of the century, guys. She's not going to really hope that Kate grows up to be a physicist.
Maud definitely doesn't approve of Kate marrying a poor nobody like Merton Densher. She tells poor Mert:
"Kate's presence, by good fortune, I marked early; Kate's presence—unluckily for you—is everything I could possibly wish […] and I've been keeping it for the comfort of my declining years." (184.108.40.206)
In a (very tasteful, very expensive) nutshell, Maud isn't all that interested in what Kate wants. She's interested in molding Kate into something that she (Maud) deems right and proper and class-appropriate. The silver lining is that at least she's totally up front and unapologetic about this.
When seen from the outside, she is an intimidating figure, but not because she ever does anything outright menacing. Rather, it seems like she always has the potential to do something harsh. As Merton observes, "Her arms of aggression, her weapons of defense, were presumably close at hand, but she left them untouched and unmentioned" (220.127.116.11). The weapons of social persuasion are there, but the fact that the old woman never brings them out is what makes her so eerily powerful. She's the English high society equivalent of a 6' 6" bouncer with his arms crossed in front of a club.
It's due to Maud's restraint from using her power that this book doesn't paint Maud as an out-and-out baddie. The novel is actually pretty sympathetic toward her. She has certain ideas about how the world should be, but she's not mean about it. In fact, she personally likes Merton Densher a lot. And let's not forget that she's helping Kate out of a really sticky family situation. That's actually fairly bighearted of her.
Maud is just super committed to doing what she thinks is right. This makes it very hard to change her mind about things, like allowing Kate to marry a penniless Merton. In certain situations Maud's commitment can also be an admirable quality, like when she tries to do a favor to a dying Milly Theale by getting Merton to visit her. Or even by forbidding Kate to see her dirtbag greedyguts of a father: this could be read as a really valid move, like getting an abused puppy out of the home that abused it.