an URL is a Uniform Resource Locator to use the full name (not important but you did ask what it was haha)
Basically it is the address on the internet where something is. Since you are referring to pictures I can give you an example:
http://img.photobucket.com/albums/v441/tho9900/P-26/engineII.jpg
is the URL the picture of the engine of the plane I am building right now...
to post pics here you need a hosting service, www.photobucket.com is a good one a lot of people use. When you post a pic to it, it automatically makes the URL at the bottom of the pic for you so you can just right click on it, click copy and then go here and click pate and it shows up!!!
Make sure when you copy the URL you use the bottom one shown on photobucket as it already has the tags you need to display the pic. (the pics here need [url.] ... [/url.] around them (without the periods) to be able to see the pic like this:
[IMG.]http://img.photobucket.com/albums/v441/tho9900/P-26/engineII.jpg[/IMG.]
is what the URL looks like for that (without the periods)