What type of figurative language is this: "His stem stood up like an antenna"?

"His stem stood up like an antenna" is an example of a simile. A simile is a type of figurative language in which two things are directly compared to each other through the use of a connecting word such as "like" or "as"—in this case "his stem" is like "an antenna."
Similes are akin to but distinct from metaphors, another type of figurative language, in that similes always use these kinds of connecting words to make a comparison, while metaphors do not, instead comparing two things directly by implying, figuratively, that one is the other. For example, to express this same sentiment via metaphor one could revise the sentence to state: "His stem, an antenna, stood up," or "His stem stood up, becoming an antenna."
Similes are frequently used as tools to enhance imagery. The imagery created by this sentence is of an antenna standing at a ninety degree angle in the air, as at the top of a retro tv set or radio.