Base64 is an encoding system that allows the translation of binary data into ASCII text strings, basing upon 64 different ASCII characters.
Each Base64 digit represents exactly 6 bits of data so, for example, six bytes (a total of 48 bits) can be represented by eight Base64 digits. By writing the command base64 you can encode and decode Base64 data. The command follows the structure: 
base64 [flag] [-i input item] [-o output item]

Defining input and output files

In this case, flag such as -i and -o are fundamental to define respectevely from which file take the stream and into which to store the result. Naturally, the path and the name of the files must be specified. 
base64 -i path/input.file -o path/output.file

Insert line breaks

Using the -b (which stands for break) flag followed by a number, line breaks are added every specified "number" characters. 
base64 -b number path/input.file path/output.file
By leaving the number field empty, an unbroken stream will be generated.

Decode Base64

Using the -D flag (which stands for decode), you obtain as output the input message decoded into binary data. 
base64 -D [-i input item] [-o output item]

Summarize the tools

Using the -h (which stands for help) a short paragraph in which are listed the flags with the respective paragraph will be displayed. 
base64 -h