What does mean in computer science?

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This has effectively nothing to do with computer science in general.

It’s just the structure of an Extensible Markup Language (XML) closing tag.
XML is a format based on tags, and HTML, for displaying web pages, is a derivative of XML.

Tags are used to delimit a piece of information and giving it the meaning you want, or in HTML, the meaning the tag is meant for (there are a limited number of predetermined tags – but you can create ones, without entering in the details).

An HTML example:

  1.  

  2. «P» is for «Paragraph» 
  3.  
  4.  
    •  
  7. «UL» is for «Unordered List» 
  9. while «LI» is for «List Item»  
  11. So this is how we represent a simple bullet-points list
  12.  
  13.  
  14.  

  15.  

  16. In HTML you also have «void elements/tags» that don’t have closing delimiter, because they have no content. 
  17.  
  18. Here is a blank line insertion in HTML5:  
  19.  
  20. Or in previous HTML versions: 
  21.  
  22.  
  23. An image:
     
  24. cat.gif  
  25.  

That’s why you can sometimes see people writing things like :

  1. HTML is great! No, that’s degenerate XML. 

When they’re afraid their sense of irony isn’t understood.

Also, the symbol > alone doesn’t mean anything and isn’t valid HTML, but is often used as a “logo” for HTML. Like on a button that gives you HTML code for integrating a video in your website or blog, for example.