Samuel Johnson, who came up with the term "metaphysical" to describe seventeenth-century poets like Donne, stated that in their poems,
The most heterogeneous ideas are yoked by violence together; nature and art are ransacked for illustrations, comparisons, and allusions; their learning instructs, and their subtlety surprises; but the reader commonly thinks his improvement dearly bought, and, though he sometimes admires, is seldom pleased.
What Johnson means here is that this group of poets groped to find striking metaphors or images that moved beyond the smooth commonplaces of Elizabethan poetry. It had become hackneyed and worn out, for example, to compare your lover's cheeks to roses or hair to gold wires, so the metaphysicals worked to come up with more interesting words and ideas. They deliberately tried to put together ideas that wouldn't, at first glance, seem to fit together--this was done to surprise people and make them think. Johnson grudgingly admits this strategy works--their learning instructs and their technique surprises, but in his opinion, the reader has to work too hard at understanding what the poems mean for the reward to be worth the effort.
Donne fits Johnson's description of a metaphysical poet. If we use one section of a famous Donne poem, "A Valediction Forbidding Mourning," we can illustrate how a startling and unusual metaphor yokes together an idea:
If they be two, they are two so / As stiff twin compasses are two; / Thy soul, the fix'd foot, makes no show / To move, but doth, if th' other do. / And though it in the centre sit, / Yet, when the other far doth roam, / It leans, and hearkens after it, / And grows erect, as that comes home. / Such wilt thou be to me, who must, / Like th' other foot, obliquely run; / Thy firmness makes my circle just / And makes me end where I begun.
In the above, Donne compares a compass—the tool with which you draw a circle—to the relationship between his lover and himself. No matter how one "foot" or leg of the compass moves, the other stays still firm and fixed and holds the moving "foot" (i.e., lover) in place. It is unusual to compare lovers to the two legs of a compass and even more so to connect that image to sexual desire: "grow erect, as that comes home." And as Johnson points out, this is not a poem one can pull the meaning out of through a quick reading—it requires work and effort. Donne uses this kind of imagery all the time in his poems, and today we have a greater appreciation of this startling technique than did Johnson.