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 hexadecimalA '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.