Enjambment occurs when a phrase carries over a line-break without a major pause. In French, the word means "straddling," which we think is a perfect way to envision an enjambed line. When you read an enjambed line, the sense of it encourages you to keep right on reading the next line, without stopping for a breather.
Or gazing on the new soft-fallen masque
Of snow upon the mountains and the moors—
The only way to make sense of those lines is to lump them together—to enjamb them. In a poem full of end-stopped lines, these two lines leap out by running together.