The top of this black thing is leaking slowly (next to shut off valve), what's it called and do(02)

The top of this black thing is leaking slowly (next to shut off valve), what's it called and do(02)

Image gallery: Leak Slowly

Slow Leak In Tire - Causes & How To Fix It? 1,026 Slow Leak Images, Stock Photos & Vectors Shutterstock The top of this black thing is leaking slowly (next to shut off valve), what's it called and do