@homeless_dingo The "f" is how big or small the aperture is opened to allow more or less light into the camera. A higher "f" value means that the "hole" is smaller allowing less light to get into your camera to the sensor. The lower the f value the larger the "hole" allowing more light into your camera sensor. These are f-stop values. The other value you are talking about (15" to 1/2000) is the shutter speed setting. It is how long the shutter is open when you take a picture. 15" is 15 seconds, 1/2000 is 1/2000th of a second. A long exposure would be 15 seconds. f 3.5 would be a large "hole" to allow more light in.
The one thing you didn't mention is ISO. The higher the number, the faster the light can be utilized by the camera (this is simple terms). I would suggest that you do some Google research on the "Exposure Triangle" which involves aperture setting (f-stop), shutter speed, and ISO. It should help to clear things up for you.