This appears to be a reference to a fan-made or unofficial compilation of Tyler, the Creator’s unreleased material, credited to a collector or compiler named “JKoop.” Tracks labeled as “unreleased” often circulate on platforms like YouTube, Reddit, or file-sharing sites, and “solid piece” suggests the user is affirming the quality of this specific compilation — meaning it’s well-sourced, properly tagged, and contains rare or high-quality leaks, demos, or loosies.