When a girl says you have a "really hot voice," it means that she finds your voice very attractive and appealing. It's a compliment on the way your voice sounds to her, and she may find it sexy, alluring, or simply very pleasant to listen to. People are drawn to different qualities in others, and for some individuals, a person's voice can be an attractive feature.
Receiving such a compliment on your voice can indicate that she has taken notice of you and is expressing her appreciation for this specific aspect of you. However, it's important to remember that a compliment on your voice doesn't necessarily imply romantic interest on its own. It could simply be a genuine observation of something she finds appealing about you.
If you're interested in exploring a potential romantic connection with her, take the compliment as a positive sign and use it as an opportunity to engage in further conversation and get to know each other better. Keep communication open and see if there are other signs of interest and compatibility beyond just the compliment on your voice.