This information is referenced in the Equations article.
Numeric equations can calculate field values, numbers, and other mathematical functions and format results in various number formats with decimals, rounding, currency, etc.

Decimals: Whether large numbers will be separated by commas, decimals, or nothing.

Decimal Places: The amount of decimal places to include when displaying the number. This doesn't affect how the numbers are stored in the database, only how they are displayed. If the stored number has more decimal places than this option, than the displayed number will be rounded.
 Rounding: Whether the number should be rounded up or down. If using the default setting, numbers ending in a number below 6 will round down, and anything else rounds up.

Thousands: Whether commas will separate thousands in large numbers. For example, 80000 would display as 80,000.

Format: Options to format your number with currency letters or your own custom before and after text.