Order
Basics
What it means
An instruction you send to your broker to buy or sell an asset. The order specifies what to trade, how much, and at what price (or at market).
Example
Sending "Buy 100 AAPL at $150 limit" is an order. The broker routes it to the exchange, which fills it only if AAPL reaches $150 or better.
Mistake beginners make
Beginners smash the buy button without choosing an order type, defaulting to a market order that can fill at any price -- often a bad one in fast markets.