He's a black man feeling very uncomfortable with the security woman following 5 steps behind him at every move. The situation escalates from there.
At first she was trying to be subtle, "pretending to shop for headphones," according to what the man says, but then she kept following him around and supposedly made a remark about how he "needed to pay for" an item he selected from the shelf.
When it became clear that she was following him around, he began filming and called her out for how uncomfortable he was making him as a customer. In response, she can be seen defiantly following just steps behind him despite how he repeatedly asks her to give him space. Frustrated, he begins to raise his voice, and the scene gets more intense and confrontational until another employee enters the conversation and agrees that the security woman should back off.