Bringing two different things together with an explicit word, such as "like", "just as" or "seems". Unlike metaphor, comparison makes plain that these are two things.
In Othello
Trifles light as air / Are to the jealous confirmations strong / As proofs of holy writ.
The phrase "as powerful as Holy Scripture" sets side by side the slight signs the jealous man sees and the sacred text. The comparison shows the weight that a mere cloth gains for someone who already wants to believe.