What does it mean in stock when the market closes or opens? What happens if it opens or closes??

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The market (ie NY stock exchange) opens at 9:30am EST and people can trade until it closes at 4pm EST.

Well when the market opens here in the USA it opens at 930 it will close at 4pm eastern time. But there is after hours trading also but thats a whole other lesson. At night the other markets around the world open & they affect the next days opening prices of our markets. Also there is that after hours trading l said above & those investors affect the opening price of the stocks as well. Also bad news in the media can affect prices as well. Market makers r people that work on the floor of every brokerage house & they make up prices for these stocks all day long this is where u get ur bid & ask prices.
But for ur question the market open & close just means the time in which NY stock exchange open & close up.
But there r other markets starting up too later in the evening.
take care...

Open for business - open market trading available

Closed until tomorrow - open market trading done for the day

That is what it means