You remember that participles act as adjectives, right? That means that they modify nouns.
Sometimes, it can be difficult to tell which noun a participial phrase is modifying. In fact, the noun that it is intended to modify may not be stated in the sentence. That's not a good thing.
When this happens, it's called a dangling participle because it just dangles there with nothing to modify.
Sitting on the park bench, the sun disappeared behind the clouds.
Sitting on the park bench is a dangling participle.
Where is the noun that sitting on the park bench modifies? It's not modifying sun or clouds. In fact, the noun that it is modifying is not even in the sentence. That poor participial phrase is just dangling there with nothing to modify. Poor thing.
To fix participles that dangle, move them so that they come right before or after the noun or pronoun that they are modifying.
Sitting on the park bench, I watched the sun disappear behind the clouds.
Now, sitting on the park bench clearly modifies the pronoun I, so it's not dangling any longer!