Compresssor shrinks the oversized videos filling up your phone down to the resolution you choose — Full HD, 2K, whatever you like — keeping HDR, colour, audio, and every scrap of metadata. It all happens on your device; nothing is ever uploaded.
4K → Full HD typically frees more than half the space, and you can barely tell the difference on a phone screen. Point it at your library and watch the gigabytes come back.