Em dashes are the width of (you guessed it) a capital M. Most often, they’re used in pairs to emphasize an element or elements within a sentence:

Cruciferous vegetables—broccoli, cabbage, and cauliflower, for instance—are said to lower the risk of cancer.

Em dashes can also show an abrupt change in thought:

I thought I had time—more than enough time—to catch the train.

Or they can show interrupted dialogue:

“I told you I can’t—”

“You mean you won’t, not can’t,” she said.