Seriously is an adverb that can be used in several ways:
1. In a serious manner: "He spoke seriously about the issue."
2. To a severe or significant degree: "She was seriously injured in the accident."
3. Used to express disbelief or surprise: "Seriously? You're kidding me!"
4. Used to emphasize a statement: "I seriously need a vacation."
5. In slang, can mean "really" or "very": "That movie was seriously good."