Caesura is a pause somewhere in the middle of a verse. Some lines have strong (easily recognizable) caesurae, which usually coincide with punctuation in the line, while others have weak ones. It's conventional to mark them with a double bar; the closest HTML allows me to come is ||.

Pope was able to keep his heroic couplets interesting by varying the position of the caesurae, as here:

Alas how changed! || What sudden horrors rise!
A naked lover || bound and bleeding lies!
Where, where was Eloise? || her voice, her hand,
Her poniard, || had opposed the dire command.

From the Guide to Literary Terms by Jack Lynch.
Please send comments to Jack Lynch.
Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.