The "Thin Red Line" flag is in support of firefighters. The company Thin Blue Line USA started making flags, T-shirts and other clothes in 2014, donating a portion of its sales to law enforcement.

NYC firefighters were were ordered to remove a "Thin Red Line" flag after receiving a complaint from a Democratic councilwoman’s office that it was a "political symbol." ...

The "thin blue line" flag is a sign of support for law enforcement that has amid Black Lives Matter protests come to signal opposition to the racial justice movement and support for white supremacy.