Why do I need resistors for each LED light when making this sign?

I came across an excellent source where it shows me how to make a portable light up sign: http://www.instructables.com/id/The-Ultimate-Sports-Fan-Sign/#step1 and of course I'm interested in making it (without the separate flashing words). But I got lost after I was told in the video at 1:29 to connect the negative ends of the LEDs together. Do I just fold them down and they're automatically connected? Secondly, I don't understand right after that step, how to connect the resistors to the positive ends and then how to connect the positive ends of the LEDs together. Why do I need specifically 33 ohm resistors and where do I get them? Do they come with LED lights? Thirdly how do I connect the positive ends and negative ends of the letters together? Finally do I need to on/off switch if I don't want the letters to separately flash? Sorry for all the questions I'm very very new to this. Please explain as thoroughly as you can. THANKS SO MUCH!