Hi. A dangling preposition is basically when you end a sentence with a preposition. Traditionally, to avoid ending the sentence with a preposition, you can use "which" or "whom" (or similar words).

Examples:

"I have a friend I'm staying with." versus: "I have a friend with whom I'm staying."

"There's some trouble I'm stuck in." versus: "There's some trouble in which I'm stuck."

This outlines the general rule. To avoid ending the sentence with a preposition, you use the preposition immediately after the object (example 1: friend; example 2: trouble), and then put "whom" for a person or "which" for a thing (trouble, table, anything other thing).

Generally speaking, I think nowadays it's pretty common for people to end sentences with prepositions colloquially; it really matters most if you're writing, especially formally (essays, articles, et cetera). And even then, it's usually not too difficult to avoid dangling prepositions, because there are usually simpler ways to phrase it (example 1: "I'm staying with a friend"; example 2: "I'm in some trouble") that keep things pretty tidy. The hardest time to avoid it is when you're asking questions:

"Who are you going with?"

"What trouble are you in?"

Hope this helps a little.