That’s oils and waxes from the bud that condense on the sides of the glass and build over time. You can clean it off by using isopropyl alcohol 99-100%. It’s also possible to reclaim it and ‘eat’ it providing potent results. Check yt for guides.
Worth mentioning that using isopropyl and eating it are almost mutually exclusive. Use everclear if you wanna safely ingest. Always research extensively when flammables are involved.