A picture or an image in a computer's memory is represented by a series of bits.
These can have a value of either ones or zeros.

11010010101000001000010000010000

We can arrange these so that every 8 bits make up one byte.

11010010 10100000 10000100 00010000

And a byte can represent the number 255 in decimal notation.

The most effective method for hiding files in steganography is normally the least significant bit method.
This type of steganography is very effective against discovery and can serve a variety of purposes.
These purposes can include authentication, concealing of messages, and transmission of encryption keys.
This simply means that the hidden message will alter the last bit of a byte in a picture.
By altering that last bit, there will be relatively no change to the colour of that pixel within the carrier image. This keeps the message from being easily detected. The best type of image file to hide information inside of is a 24 bit Bitmap.
This is due to the large file size and high quality.

In a bitmap image you have a series of bits which represent the colour intensities of that image. The hidden message will alter the last bit of a byte in that image.
In other words the bits that make up the hidden file or picture are placed in the least-significant bits of each colour of the picture carrying the hidden message. The least significant bits are the rightmost bits of the byte.