You are moving into financial terms and need to start using financial sites. You're moving beyond most simple English explanations.
When you buy on margin, you assume the price will go up.
You buy those shares worth 10,000 thinking they will be worth 12,000 in a few weeks so you'll borrow the money to buy them based on the promise to sell them.
Buying On Margin Definition
I don't know how good this site is, but it has a dictionary.
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