A simile is a figure of speech that uses "like" or "as" to compare two different things.

In this example from Edgar Allan Poe's "The Tell-Tale Heart," it's important to look at the lines that came before the simile to fully grasp the simile's true meaning:  

And have I not told you that what you mistake for madness is but over-acuteness of the sense? --now, I say, there came to my ears a low, dull, quick sound, such as a watch makes when enveloped in cotton. I knew that sound well, too. It was the beating of the old man's heart. It increased my fury, as the beating of a drum stimulates the soldier into courage.

Here, Poe compares the narrator's escalating anger at discovering the old man's heart still beating to that of a soldier's ascending courage at the sound of a military drum. The drum stimulates the soldier's courage and quells any fear or hesitation before he goes into war. Likewise, the beating sound of the old man's heart triggers the narrator's rage. Poe uses the solider-and-drum simile to illustrate the narrator's frustration at the familiar sound of the old man's beating heart.