The Internet is a big place. Theoretically, there are roughly 4.3 billion available IP addresses, though this includes reserved IP addresses and nonroutable addresses. Still, the average human can't remember a hundred or so IP addresses, let alone more than four billion!
Luckily, the Domain Name Service is there to help. DNS resolves, or looks up assignments of, Internet names to IP addresses as well as IP addresses back to names. Names are far easier than numbers for the human mind to store and retrieve. Which is easier to remember, yahoo.com or 184.108.40.206? We suspect that you could list more than a hundred domain names without thinking too hard, whereas you'd be hard pressed to list 50 IP addresses (the sequential numbers in your own network don't count). In simple terms, DNS is basically a giant phonebook for the Internet, wherein you can use a name to get an address or vice versa.
The new DNS administrator is often confused by the difference in terminology when talking about DNS servers from the server side and DNS servers from the client side. From the server side, BIND/DNS servers are referred to as master and slave. This indicates which server holds the master DNS records and which one retrieves its DNS records for DNS services. The terms primary and secondary only refer to how clients access the DNS servers (which may be backwards from the master/slave designations in some cases).