A position is the amount of an asset that an investor/trader has. You can have either a long position (where you buy and then sell), or a short position (where you borrow and then sell).

Depending on how the price of the asset moves, a position can profitable or unprofitable. Long positions make money when the price of an asset increases. Short positions make money when the price of an asset decreases.

When someone enters a trade, they open a position. When someone exits a trade, they close a position.

The terms "opening positions" and "closing positions" are used because it gets confusing to use terms like buy/sell when you can either go long or go short. Going long you buy the asset and then sell it later. For going short, you're essentially selling the asset first, and then buying it later. So the terms "opening and closing positions" are generic so as to be useable in both situations.