If something is slangier, it has more slang words in it or is more like slang.
Slangier can make people think of being more casual or informal.
Examples:
The new book is slangier than the old one.
Her speech is getting slangier every day.
The slangier words are easier to understand.
I didn't understand the slangier parts of the conversation.
The slangier the talk, the harder it is to follow.
The movie is getting slangier and slangier as it goes on.