It literally translates as 'to salute the flag'. In slang, it is used to refer to the act of making an effort that ends up being ignored or unacknowledged.
Examples
- Spanish: Hice todo el trabajo y el jefe ni siquiera lo notó. Siento que solo estoy saludando a la bandera.
- English: I did all the work and the boss didn't even notice. I feel like I'm just saluting the flag.