The decimal (base ten) number system is the most widely used number system. It uses the number ten as its base. Decimal numbers use the digits 0 through 9.
The term decimal or DEC can refer to the base ten form of an identifier, as opposed to the base sixteen (hexadecimal or HEX) form.
In Pokémon Red/Blue/Yellow, money is stored in a unique way, so that the hexadecimal value of a money byte matches the decimal value (e.g. '99' is actually 99).
Converting from to decimal to hexadecimal
A 'by hand' method of converting from decimal to hexadecimal is described here.
One can easily use a tool to convert from decimal to hexadecimal, such as this online one.
Alternatively, one can open Windows Calculator, click 'view', click 'programmer', click 'dec', enter a number, and then switch to 'hex' to convert a decimal number into hexadecimal.