They are units that are not relative to any element or body. They can be further understood as independent unit as whatever they are being attributed to doesn’t depend on anything to attribute it’s length, width or size.
They are units that are relative to their parent body or element. They can be further understood as a dependent unit as whatever they are being attributed to depends on its parent properties to determine its own properties. Elements they are applied to are always responsive.
Examples of this include: px, cm, mm, in, pt
Examples of this include: em, rem, %(percentage), vh, vw, vmin, vmax e.t.c