He means stay aware and smart. It's a military term from the Gulf Wars referring to the way in which uniforms would become "salty" with time in the desert. Being salty means you are experienced and unlikely to die in a stupid way.
I posted a misinformation. It predates the Gulf Wars, that's just the first context I heard it in.
Related: 'salty language'
Okay hopefully the Ministry of Truth doesn't ban me for this misinfo
but then again they did name this book about Marine Corp lingo after it. Worth noting that Marines are called Marines because they were stationed on ships, so both takes are kinda right.