It's important to consider the context to determine the exact meaning:
* It looks better: This translation is used when referring to an object or a situation. For example, "The house looks better after the renovation."
* He/She looks better: This translation is used when referring to a person. For example, "He looks better after his surgery."
The word "se" in this phrase is a reflexive pronoun, and "mejor" is the comparative adjective of "bueno" (good).