You learned 原 means "original," but another meaning for it is "field." Combine grass and field and you get… a grass field!

草原 is for all kinds of grass fields: little local ones, medium-sized meadows, and even vast grasslands like the prairie or savanna.

This jukugo word uses on'yomi readings for the kanji. The そう reading for 草 is new for you, so here's a mnemonic:

When you die, you go to a grass field full of souls (そう). Idyllic image of heaven, right? Kind of like the ending scene in Gladiator, except it's not just the main character's soul, it's you and a billion other souls rubbing elbows in a grass field.

Note: The main そうげん reading is used for wide grassy plains and grasslands. The くさはら reading feels smaller and more local, like a patch of tall grass or weeds where kids run around, catch bugs, and occasionally lose a baseball forever.