You can use binary digits to represent denary values, and can therefore add them together. In this situation you must add each digit from right to left!!!
When adding two 1s, you must set the value to 0 and carry the 1 to the next column because there is no such thing as the value 2 in binary. However, sometimes there is a problem when adding binary of limited digits (ie 4 or 8). If we run out of room for the overflowing value to be carried to, it can cause a miscalculation called an Overflow Error