A very noob question

What does the Close operation on the MetaTrader 4 mean? I'm a beginner in forex but I have a bit of experience in stocks. In stocks I only knew of BUY and SELL, but in forex there is a CLOSE. Can anyone tell me the theory behind this? Do I need to CLOSE after I buy a currency or do I need to SELL that currency to gain profits? Thanks in advance.

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