DNS (Domain name system) provides a way to discover servers using a user friendly name, for example google.com. You can think of DNS as a digital phonebook, that allows you to lookup the name of a service and obtain an IP address you can use to communicate with it. Each hostname can provide multiple "records", which comprise of both a type and value.
The most typical use of DNS is to obtain A/AAAA records, which provides a user with an IP address for a hostname. Unfortunetly, this does not allow you to provide the network port to the user. This is an issue when multiple services/servers are hosted at the same IP address.
SRV records provide a solution to this, by specifying a port, address and the type of service provided. This allows you to connect to a hostname on Minecraft, and the game to connect to the correct port.