A garden path sentence leads a reader to interpret it one way only to learn later that the interpretation was incorrect. In other words, you teased them.
For example, take the following sentence:
Fat people eat accumulates.
When most people read this sentence, they assume that “fat” is an adjective describing what type of people are eating in the sentence. That assumption works right up until the last word. The reader expects an object that fat people eat. When they see a verb, it throws them off.
In the example, “fat” is actually a noun. In fact, it’s the subject of the sentence. What if we added the word “that”:
Fat that people eat accumulates.
Now, the meaning is much clearer.
Fixing a garden path sentence, however, isn’t always as simple as adding “that”. Take a look at another sentence.
The dog that I had really loved bones.
In this one, the reader assumes that “really loved” describes the relationship between the speaker and the dog. In reality, it describes how the dog feels about bones. You can’t fix it by simply adding “that”. Actually, it already contains “that”. This one would take a complete rewrite:
My old dog really loved bones.
Keep in mind that garden path sentences aren’t grammatically incorrect; they’re just confusing. And if you’re interested in clear, concise messages, try to avoid them.