The Binary (Base 2) Numeral System uses the digits 0 & 1.
The following table shows the first 8 binary values, along with the equivalent decimal values:
Binary  1  10  11  100  101  110  111  1000 
Decimal  1  2  3  4  5  6  7  8 
For further information on the binary numeral system, see the Wikipedia Binary Page
The Excel Dec2Bin function converts a decimal number into a Binary (Base 2) number.
The syntax of the function is:
Where the function arguments are as follows:
number    The decimal number that is to be converted to binary. 
[places]   
An optional argument, which specifies the number of characters that you want the returned binary number to have. If this is greater than the minimum, the binary number will be padded out using leading zeros. If omitted, the returned binary uses the minimum number of places. 
The following spreadsheets show five examples of the Excel Dec2Bin function.
Formulas:

Results:

Note that, in the above example spreadsheet, the negative binary number in cell A4 is represented by two's complement notation.
Further details and examples of the Excel Dec2Bin function are provided on the Microsoft Office website.
If you get an error from the Excel Dec2Bin function this is likely to be one of the following:
#NUM!   
Occurs if either:

#VALUE!    Occurs if one or both of the supplied number or the supplied [places] are nonnumeric. 