How to tell?

Hi all I had new and sensitive problem with my 5 years old son who is in senior kindergarten. Actually he is very good boy and always he will listen my words. But today while coming from school in the bus he kissed another girl. After coming home suddenly he is crying alot I did'nt understood what happend. I asked him gently what happend and why u are crying. Then he is telling that I kissed Mari( his friend's name) in the school bus and everybody yelled at me, and everybody said YOU to me he said. I didn't understand what is YOU then he said thats a yucky word. What ever he did something wrong and everybody laughed at him. Now I am not undersatnding that how to expalin that its wrong and u are not supposed to do that............I am confused and worried about this issue. Can u please help me in this. Thanks in advance.