Dedicated SSL is when you have an SSL certificate for your domain. In order for people to access files on your site securely, they would point their browser to https://mydomain.com (instead of http://mydomain.com - only the "s" gets added to "http").
What is SSL in general?
SSL is like secret codes you made up in Elementary school.
You take the alphabet, and make each letter equivalent to a random number. You know the code, and your friend knows the code, but nobody else does. So you can write out a note and then convert it using your code, and pass it to your friend, and nobody can figure it out without the code.
But of course they can if the note is long enough and they're smart enough. So you make the code more complicated. For example, every other letter uses a different code. Something like that.
Well, take that complexity and multiply it by several thousand and you've got SSL. Your browser and the server know the code, so when they send info to each other they each can read it, but if anyone in between intercepts the message, they can't decode it.