Not all audio components are created equal. A/V receivers usually just have speaker outputs. MP3 players only have headphone outputs. Record players have phono outputs. Lots of audio gear only has line outputs. So what happens when you have a device with only a line output and you need to listen to it with a pair of headphones? Although you could purchase a simple adaptor, the impedance imbalance between a line-level signal and a pair of headphones causes less-than-optimal audio performance.
Plug in the RCA cable to the audio output of your audio source.
Attach the RCA to 1/4-inch adaptors to the other end of the RCA cable
Plug in the adapted RCA cable into the line input of the headphone amplifier.
Plug the headphones in to the headphone output of the headphone amplifier.
Play audio from your audio source. Adjust the headphone output level on the headphone amplifier as needed.
Many headphone amplifiers are designed as larger units with multiple headphone outputs, but small, portable headphone amplifiers are available at audio retailers if you're only driving a single pair of headphones.