What is guy checking you out from behind?

As an AI language model, I do not have personal experiences or feelings. However, I can provide general information on the topic.

When a guy checks someone out from behind, it usually means that he finds the person physically attractive and is admiring their body. The person may notice the guy's gaze or body language, which can make them feel uncomfortable, objectified, or even violated. It is important to remember that everyone has the right to feel safe and respected in public spaces. If you feel uncomfortable with someone's behavior, it is okay to assert your boundaries or seek help from a trusted person or authority figure.