The acronym MAC address stands for media access control address. A MAC address is a unique identifier for your network interfaces. For example, on your computer, your wireless card and your Ethernet card will each have its own unique MAC address.
These days, Wi-Fi is becoming more and more common. Due to this, stealing Wi-Fi is also becoming more of an issue. If you have a wireless router in your home, you will definitely want to secure your network by adding a network password key to your router. When you do this, I would recommend using WPA over WEP as well. WEP is much easier to hack when compared to WPA. On top of this, I would suggest setting up MAC address filtering. The reason I suggest this is because hackers are getting better and better at cracking into Wi-Fi networks. At this point, WPA is generally considered pretty safe. However, WEP was considered pretty safe at one point and nowadays and good hacker can crack into a WEP network in a few minutes. By adding MAC address filtering, you add an extra layer of protection to your Wi-Fi network.