Every price on a chart is the last price at which a buyer and a seller agreed to trade. Nothing more. Price rises when buyers are more eager than sellers, and falls when sellers are more eager than buyers.
Bid, ask and spread
At any moment there are two prices: • The bid: the highest price a buyer will pay right now. • The ask: the lowest price a seller will accept right now. The gap between them is the spread. Buy at market and you pay the ask; sell at market and you get the bid. So every trade starts slightly in the red, by the spread.
Liquidity
Liquidity is how much can trade near the current price without moving it. Large stocks, major currency pairs and index futures are very liquid: tight spreads, fast fills. Small caps and thin altcoins are not: wide spreads, and your own order can push the price against you. While you're learning, stick to liquid markets. Your costs are lower and the charts behave more cleanly.
Someone is on the other side
For every buyer there is a seller. When you buy, someone else decided that selling at that exact price was a good idea. Remember that whenever a trade feels too obvious.
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