The era of stable Spotify mods is ending. Spotify recently introduced "Server-Side Feature Flags." This means even if you download a modded APK, Spotify’s servers can turn off your "Premium" features remotely within 24 hours.

Therefore, is almost certainly a colloquial term for a modified (modded) version of the official Spotify APK that unlocks Premium features: unlimited skips, no ads, and offline downloads (though offline rarely works in mods).

You don't actually need a "Mod." The already does 95% of what a cracked Spotify client promises. The "Spotify X" label is mostly marketing hype used by repackers to drive downloads.