Answer:
With an equalizer you are able to adjust the volume levels of various ranges in the audio spectrum. This means that you are able to, for example, increase the lower frequencies. This will increase the amount of bass you hear. The same goes for other frequencies along the spectrum, from low to high. It's recommended to use built-in equalizer presets. Below you can see a screenshot from the equalizer in the iOS app including presets:
iOS:
The equalizer can be accessed by default on all iOS devices. Here's how to access the Equalizer on iOS:
Android:
On Android devices you'll have the equalizer enabled within the Spotify Settings only if your devices' manufacturer has enabled you to access this feature. Here's how to adjust the Equalizer on Android if you have this option available:
Windows and MacOS:
Keep in mind that the equalizer can't be accessed when streaming via Connect. In this case we recommend finding out if the speaker/receiver you are streaming to, has a native equalizer built in the settings.
You can find more info about similar features like Audio Quality and Volume Normalization on our support page here.