Nadia: Wow, you're dressed up for work. What's the occasion?
Phillip: I'm going to my friend's wedding this afternoon, so I figured I'd just wear my suit to work rather than change later.
Nadia: Well, you look very nice. You should dress up more often.
Phillip: Thanks. I do feel nice.
Meaning: The phrasal verb "dress up" means to dress in clothing you wouldn't normally wear. It can be used to say someone is wearingn formal clothes, as in the example above, where Phillip is "dressed up" in a suit for a wedding. In addition, you can use "dress up" when a person puts on a costume, such as for Halloween, as in the next example:
Maria: Are you going to dress up for the Halloween party?
Neil: Nah. Wearing costumes is for kids.
Maria: What are you talking about? I love dressing up. My friends and I plan out our costumes every year. We have so much fun.
Neil: Huh, maybe I should reconsider then. What are you dressing up as this year?
Maria: I can't tell you! It's a surprise! But you should dress up as Dracula! You kind of look like Bela Legosi.
Meaning: In this case, "dress up" is used for costumes. Notice that "as" can be added to "dress up" when specifying what/who someone is dressing up as. In this example, Maria won't say what she is dressing up as, but she says Neil that Neil should dress up as Dracula since he looks like the actor who played Dracula.