Hot linking is if you use a website that isn't either your own *OR* an image host and link to it.
For example if I was to link a picture of a graphics card from OCUK's store and put it on a website I have as a "review of graphic card" picture.
The reason for this is because of a lot of bad experiences websites have given OCUK forums in the past with hotlinks, including but not limited to very disturbing / sick porn images.
*EDIT*
Oh, if you find an image that you want to post go to imageshack or another such website and upload it there. It is perfectly fine to post (as long isn't rude or offensive). BTW, if you are going to use a personal website that you own in the post you might want to mention for the first few times that it's yours so mods know.