What does the expression "I feel you" mean?

"I feel you!" When can we use this sentence? When we want to say "I understand why you are sad..." or "I understand you or I feel your pain or any other emotion." The closest and simplest meaning for it is "I understand", and we usually use it when someone is talking about something that has caused her sadness, frustration, etc. By saying "I feel you" we show our sympathy to them.

Source: Amazing Talker