Idea of Adjust Icon
It's an image of three vertical analog equalizer sliders arranged side by side. The slider knobs are all in different positions vertically to indicate different selection of base, mid, and treble. It is three vertical lines each with one rectangular knob.
This Adjust icon is made in Material Design style
developed by Google. This style is based on extremely simplified shapes, thin two-pixel lines, and is optimized for 24x24 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 6,200 icons, all in the same style and quality.