A Digital Tuner is what allows your TV to receive digital television signals (as opposed to analog channels) that are broadcast over the air.
Most of the world has now transitioned to digital television since it enables broadcasters to club multiple channels on the same frequencies. Hence many HDTVs and SmartTVs these days come with an integrated digital tuner. You can check for labels like integrated digital tuner, digital receiver, ATSC tuner, DVB tuner.
In case you have an older analog TV without an integrated digital tuner, you can use an appropriate set-top box or other DTA (digital to analog) convertor to view digital channels.