Proficiency dictates the level of your weapons and power weapons in Saros.
Here's everything you need to know about proficiency in Saros.
Saros proficiency, explained
Your proficiency level can be found in the bottom left corner of your screen, and it determines the level of weapons found on your runs and the level of your starting weapon. For example, if your proficiency level is 10, your starting weapon and power weapon will be level 10 and weapons you find will be level 10 or higher.
Graphic: Johnny Yu/Polygon | Source images: Housemarque/Sony Interactive Entertainment via Polygon
The reason you want to improve your proficiency is not only because bigger number means better gun (somewhat), but because higher level weapons come with more traits. Early on, your weapons may come with one or two traits, but, at higher levels, your weapons will have five traits. Additionally, your weapons will have a higher maximum stat cap, meaning your attributes will have more of an effect on your weapons.
There are two ways to increase your proficiency level: picking up lucenite or unlocking profiency enhancement nodes in the Armor Matrix. Your proficiency level will naturally increases during your runs because you'll defeat enemies and, in turn, pick up the lucenite that they drop. However, if you want to get your proficiency even higher, make sure to go down every side path, defeat every enemy you find, and quickly pick up the dropped lucenite.
Graphic: Johnny Yu/Polygon | Source images: Housemarque/Sony Interactive Entertainment via Polygon
To see how close you are to leveling up your proficiency, take a close look at the bar between your proficiency level and integrity. As you gain more lucenite, the bar will fill up, and, when it's full, your level will go up and the bar will empty out.
Graphic: Johnny Yu/Polygon | Source images: Housemarque/Sony Interactive Entertainment via Polygon
As for the Armor Matrix, there are 25 proficiency enhancement nodes that you can unlock, meaning the highest level your starting weapon can be is 25. There are five nodes before the first boss, so make sure to grab them.