This translates the signal into something the television can understand and display.
Check out the ports on the back/side of your television.
A DVI input looks like what older computer monitors used to connect.

Plug the HDMI cable into the PS4 and the other end into the HDMI converter.
Switch your television to the proper input and turn on the PS4.
If you see the Sony logo on screen, then you know it worked.

You might not always need a converter.
Some third-party companies produce HDMI-to-composite cables that do not use a converter.
The quality cannot be guaranteed with these cables, and you may experience audio issues.

Plug the HDMI cable into the PS4 and then plug the other end into the DVI converter.
Switch your display to the proper input and turn on the PS4.
If you see the Sony logo, then it worked.
Turn up the volume and test whether sound comes through.
If neither of these methods work, consider purchasing a new television.
Although effective, the methods listed above are not guaranteed to work every time.