When Isabella believes that she has fallen in love with Heathcliff, Catherine and Nelly try to warn Isabella about Heathcliff's true nature, but she doesn't heed them. Catherine uses nature imagery to convey that Isabella would be completely at Heathcliff's mercy.
"Tell her what Heathcliff is—an unreclaimed creature, without refinement—without cultivation; an arid wilderness of furze and whinstone. I'd as soon put that little canary into the park on a winter's day as recommend you to bestow your heart on him! [...] [H]e'd crush you, like a sparrow's egg, Isabella, if he found you a troublesome charge."
In particular, Catherine uses imagery associated with tame birds (a pet canary that couldn't fend for itself outdoors) or defenseless eggs (sparrow eggs are among the tiniest of wild birds' eggs) to warn Isabella that Heathcliff will treat her cruelly, and there will be nothing she can do to protect herself from him. In the same chapter, Nelly, too, advises Isabella to forget about Heathcliff: "He's a bird of bad omen; no mate for you." "Bird of bad omen" is an idiom that means Heathcliff is bad news, auguring a bad future for Isabella. In her more colloquial tone, Nelly builds on Catherine's more picturesque imagery to make clear that Heathcliff isn't the type of "bird" to match the delicate Isabella.
There's additional nature imagery in this passage: Catherine also associates Heathcliff with the wild, uncultivated Yorkshire moors. "Furze" refers to an evergreen shrub (gorse) that's ubiquitous on the moors; whinstone refers to dark, massive rocks common in the North of England. By contrast, Isabella has always been sheltered at Thrushcross Grange and carefully cultivated as a gentleman's daughter. She and Heathcliff are a complete mismatch, and unlike Catherine (whose choice of words suggests she's drawn to the wild moors, like Heathcliff), Isabella can't stand up to his rough, "unreclaimed" nature.
Dramatic irony is also at work in this passage, as readers know that Heathcliff is as bad as Catherine and Nelly say, but must watch Isabella naively blunder into a doomed marriage in the chapters ahead.