Watch any live chart and the last price flickers: 1.08504, 1.08507, 1.08505. It looks like a line wiggling on its own. It is actually a ledger. Every tick you have ever seen was a transaction: a real buyer and a real seller agreeing on a price at a moment in time. Nobody moves price by decree, not banks, not brokers, not news anchors. Price moves because orders meet other orders, and once you can picture that meeting, charts stop looking like magic and start looking like mechanics.
Two prices, not one
There is no such thing as the price of EURUSD. There are two: the bid, the highest price any buyer currently offers to pay, say 1.08504, and the ask, the lowest price any seller will currently accept, say 1.08512. The gap between them, 0.8 pips here, is the spread. When you buy, you pay the ask. When you sell, you receive the bid. That is why every position opens slightly in the red: buy at 1.08512 and the position is marked against a bid of 1.08504, so you start 0.8 pips down before anything happens. Most charts draw the bid line only, which is worth knowing the first time a sell stop fills at a price your chart never printed.
The two orders that do everything
Every order type on your platform reduces to two behaviors. A limit order waits: I will buy, but only at 1.08490 or better. Waiting orders stack up at each price level and form the order book, the visible queue of intentions above and below the current price. A market order acts: buy now, at whatever the best available price is. Market orders do not wait, they consume the waiting orders. Stop orders, the kind attached to your trades, are simply triggers: when price touches the stop level, a market order fires. That detail matters, because it means a stop guarantees an exit but not an exit price.
How a tick actually happens
Picture the book: sellers waiting at 1.08512 with 25 lots, more at 1.08513 with 40 lots, buyers waiting below at 1.08504. Now market buy orders arrive for 25 lots total. The entire queue at 1.08512 is consumed. The best available ask is now 1.08513, and the chart ticks up. That is the whole mechanism: price did not rise because something abstract happened, it rose because buyers exhausted every seller at the old price. When aggressive buying keeps outrunning the sellers level after level, you get a trend. And price can move with no trade at all: if those sellers simply cancel their 1.08512 orders and requote higher, the ask jumps without a single transaction printing.
Thin books, fast moves and slippage
The book is not constant. In the seconds before a major data release, market makers cancel their resting orders to avoid being run over, and the queues thin dramatically. A market order arriving then chews through several sparse levels at once, filling parts of the order at progressively worse prices. That is slippage, and it is also why a news candle can jump 30 pips in a second: there was simply nobody waiting in between. Your stop loss lives by the same rules. Triggered during a fast move, it becomes a market order eating whatever liquidity exists, which can be 2, 5, or 15 pips beyond the level you chose. Slippage is not your broker cheating. It is the book being empty where you assumed it would be full.
What a beginner should do with this
Four practical consequences. Trade liquid instruments in liquid hours, because a thick book means small slippage and honest fills. Watch the spread as a live gauge: when it suddenly widens, liquidity is leaving and your costs just rose. Never hold a tight stop through a scheduled red-calendar release, because the stop will trigger, but the fill can land far beyond it. And treat limit entries as a discount tool: entering where others are rushing means paying the spread plus slippage, while entering with a resting order at your level means the market comes to you.