In Forex Trading you'll often hear position size referred to as a number of Lots.
If you're familiar with derivative trading in crypto, you've probably dealt with "contracts" before.
A Lot is similar to a contract - both refer to a unit a numerical measurement of an asset.
These are known as quantifiers, like a dozen eggs. Dozen just represents 12.
Essentially, a Lot is a number of units.
A Standard Lot is 100,000 units of the base currency (first in the pair).
So for EURUSD, 1 Lot is 100,000 Euro.
For USDJPY, 1 lot is 100,000 USD.
You can also deal in partial lots.
A mini lot is 10,000 units or 0.1 Lots.
A micro lot is 1,000 units, or 0.01 Lots (The minimum tradeable size on a number of brokers)
Other Asset Types
The other assets are not as generalised in their lot sizes. Different brokers may use different values for a lot, and even across one asset type the same broker may use different values.