Break out in hives?

Dear users,

Here's a quote from Robert Plant:

Referring to the band’s 2007 reunion, Plant said that “the idea of doing it next summer and the summer after that and so on is enough to make me break out in hives.”

source: http://ledzepnews.com/2019/05/21/robert-plant-said-that-he-loves-jimmy-page-and-wants-to-hug-him/

I'm wondering what break out in hives means and in what contexts I could use it.

Thank you!