What is GET, when you get a weapon? What is secret info for in career?

Sometimes I get a "GET" message when I obtain a weapon, what does it mean? and what does secret info say, i get that there are different things it says but besides the one's that highlight an area of the map I don't understand what they mean. What is secret info for anyway there's even an item (double notebooks) that makes you get more during a mission but what's the point.

I was told that it means you got that weapon for achieving a large number of objectives on the level and that this weapon has higher stats than usual