A decimal number can be defined as a number whose whole number part and the fractional part is separated by a decimal point.

Explanation: Let us discuss decimal.
A number whose decimal part and whole number part are separated by a point is called decimal.

For example, \$673.921\$.
Here, \$673\$ and \$921\$ are separated by a point. So, \$673.921\$ is a decimal number.

Subtracting decimals means finding the difference between decimal numbers.

Explanation: In this image we are subtracting two decimal numbers \$99.99\$ from \$86.33\$ we get \$13.66\$.