Can someone explain me these 3 terms

[instaleech]

I am interesting in learning to secure my forum but I cannot completely understand the following terms

• Low Risk
  
• Medium Rik
  
• High Risk 
  

Whenever I read them on bug fixes I cannot understand if for example a low risk sqli has been found does it mean that the attacker has a low change of SQLing my site. etc... for medium, high change of hacking into my site.
Am I in the wrong direction do those terms mean anything else.

[Nathan Malcolm]

Low risk usually means additional access is required, e.g. access to the Admin CP, or while it doesn't pose a direct threat it could aid an attacker in exploiting your forum (e.g. Full path disclosure).

Medium risk means a significant issue which needs to be patched or it could put your forum or users at risk.

High risk means you're pretty much screwed if you don't upgraded ASAP.

Hint: Always upgrade to the latest stable release of the series.