Look at that outer circle. Your submarine's bow is pointed in the direction of 0, so it's going left on that picture: <------<.
If you now spot a target 'bearing 25°', that means it's currently at 25° on that outer circle, so a bit to your right.
If you spot a target bearing 335° it's a bit to your left as you can see. The degrees are always counted clockwise starting from 0.
Now keep this in mind
. All of this is about relative bearing. Relative to what? To your submarine's bow, which is at 0°.
Introducing 'course': a ship is moving on a certain course, again expressed in degrees. Take that same circle, but rotate it clockwise one quarter so that the 0 on the outer circle is pointing upwards (north). Now you can see what each course means. If your submarine is on course 0°, it's going north (up). If it's on course 90° it's going due east, 180° south and 270° west. If a ship is reported going north-east, it's moving at a course of approximately 45° (up and right, diagonally). Don't forget, 0° is always pointing north (up) when talking about course, while 0° is always pointing to your submarine's bow when you're talking about bearing.
Now combine the two: your submarine might be on a
course of 90° (due east >---->), and spot a target at
bearing 270°. That means that when you look north (which is to the left of your bow), you'll see that target.