If a girl says you have a "really hot voice," it typically means that she finds your voice very attractive and appealing. People's voices can have various qualities that others may find attractive, such as being deep, soothing, confident, or having a pleasant tone. When someone compliments your voice in this way, they are expressing their admiration and attraction to how your voice sounds.
It's important to remember that compliments about someone's voice can be subjective and based on personal preferences. Different individuals may have diverse tastes when it comes to what they find attractive in a voice. Nonetheless, if someone specifically mentions that your voice is "hot," it is generally regarded as a positive and flattering remark.