Liquidity
Basics
What it means
How easily you can buy or sell an asset without moving its price. High liquidity means large orders fill with little price impact.
Example
EUR/USD is so liquid that a $1 million order barely moves the price. The same order in a small altcoin might push the price up 15% before it fills.
Mistake beginners make
Beginners buy low-liquidity coins because they look cheap and "move a lot," then cannot exit without crashing the price against themselves.