An end-stop occurs when a line of poetry ends with a period or definite punctuation mark, such as a colon. When lines are end-stopped, each line is its own phrase or unit of syntax. So when you read an end-stopped line, you'll naturally pause. In that sense, it's the opposite of enjambment, which will encourage you to move right along to the next line without pausing.
Bright Star, would I were as stedfast as thou art—
Not in lone splendor hung aloft the night,
And watching, with eternal lids apart,
Like nature's patient, sleepless Eremite,
See how the lines each have their own units of sense? And they each end in a punctuation mark that indicates a pause. These, Shmoopers, are end-stopped lines. In fact, "Bright Star" is mostly end-stopped, with only two examples of enjambment. Check out the poem, and see if you can find them.