What is added when extra filler data is needed in a packet?

In Computer networking, you have data that is transmitted through packets. In which is what we call an Ethernet frame. That Ethernet frame broken up into several pieces. They are:

1. The Preamble
2. Recipient's MAC
3. Sender MAC
4. Type
5. Data (Your information)
6. Pad (FILLER)
7. FCS

The reason why we have the "Pad" is because your standard minimum for an Ethernet frame size is 64 bytes. However, it doesn't all have to be your actual data. For example, if you were to send data that was not 64 bytes. Your NIC (Network Interface Card) will automatically add extra data which is what we can the "Pad" portion. This adds data so that we are back up to the minimum value of 64 bytes within an Ethernet frame.