Friendship_Cannon's post in URL was marked as the answer
an URL is simply the webadress of a website...
You can see it on the top of your page, in that open line, it usually looks like this:
http://mlpforums.com/topic/61887-url/#entry1502695 (this is the webadress of this topic, just as an example)
You can left-click this line up on your screen to mark it and then you can "copy" it with the left-click and "paste" it where you wanna (again with left-click).
If it is an URL of a picture:
when you write a comment theer are different settings and options for hwo you wanna write. There, too, is one thing (left under the smiley face) that looks like a small photo, if you click this it tells you to enter an URL.
If you paste the URL you copied before there, that will make the picture appear in your post.
If it is an URL of a website or an youtube video:
just paste the URL you copied from the top of your screen right in your post.
If this URL is from a youtube video this video will appear in your post,
if it is the URL of a website the webadress will appear as a link that will lead everyone who presses it to the website.
Friendship_Cannon's post in URL was marked as the answer