Tongue twisters are phrases designed to be challenging to pronounce. Getting tongue twisted is funny, and part of that fun comes from seeing how fast you can pronounce the tongue twister. Don't believe me? Try saying this super famous English tongue twister:
How much wood would a woodchuck chuck if a woodchuck could chuck wood?
Enjoying the challenge?
Which wristwatches are Swiss wristwatches?
Freshly-fried flying fish drop and flop.
She sells seashells by the seashore.
A proper copper coffee pot.
My mom's mom made it a mission to memorize macros.
You see, even for native speakers, these phrases cause us to "trip" over our words. They're really tough to pronounce, but, as you've noticed by now, they're also amusing.
Now, what does this have to do with improving your English pronunciation? Well, tongue twisters are difficult to pronounce because you have to move your tongue and mouth very quickly in ways you aren't used to. You can think of learning English as a parallel. Your mouth and tongue aren't used to making English sounds, and so sometimes you get tripped up speaking or reading English. This doesn't mean you are a bad English speaker; it means you need more practice.
Additionally, you probably don't know how to make every sound in English properly. In order to produce sounds correctly, you need to know where your tongue goes in your mouth and what shape your mouth makes (when you practice, you'll need to use a mirror!). You also need to know if sounds are voiced or unvoiced. Skype English Class Now offers videos and written guides that teach you how to make English sounds: