Exposure is the combination of shutter speed, or the duration light is allowed to enter the camera, and aperture, or the volume of light entering the camera. Much like filling a glass of water from a tap, you can open the tap all the way (a wide aperture), and the glass will fill quickly, or you can just slightly open the tap (small aperture), and the glass will fill slowly (long shutter speed). Or some combination in between. The idea is to fill the glass.
Exposure = Shutter Speed/Aperture
...or Exposure = Aperture/Shutter Speed. To get a certain exposure value, aperture and shutter sped are combined.
The exposure needed for a scene combines the amount of light in the scene and the sensitivity (speed) of the film being used. Therefore, the light needs to be measured, or metered. Briefly, the meter does the combining of film speed and available light to present an exposure solution (more on meters and metering in Meters and Metering). This solution is set of combinations of shutter speed and aperture, all of which will add up to the same amount of light hitting the film.
So, walking outside and taking a meter reading gives the following set of equivalent exposures:
Shutter speed and aperture have an inverse relationship. If you make one greater, the other needs to be reduced by the same amount. The choice is determined by the subject. Shutter speed becomes dominant when the subject is in motion. Aperture becomes dominant when near-to-far focus (depth of field) becomes important.
Both shutter speed and aperture are measured in stops. Each stop equals a doubling or halving of the previous value. So moving the shutter speed dial or the aperture ring one click equals one stop. As an example, the meters reads ƒ/8 at 1/250th of a second. If you set the shutter speed to 1/125 (one stop longer) the aperture needs to be set to ƒ/11 (one stop smaller), to keep the equivalent exposure.
Accurate metering and exposure are paramount to creating successful images. It is difficult if not impossible to make a good print from a poor negative. Conversely, the better the negative, the easier it is to make a good print. Although black and white film in particular has a fairly wide exposure latitude, careful metering and exposure is key.
With black-and-white film and its exposure latitude (tolerance for under- and overexposure), many photographers regularly employ bracketing when making exposures. This technique uses a series of exposures, over and under the metered, 'correct' exposure, to ensure getting a good negative. Depending on the camera, the sequence may be in full stops (2 under, 1 under, on, 1 over, 2 over) or 1/3 stops. Five frames is usually enough. Although bracketing becomes the shotgun method of exposure, it does work. Film is generally less expensive than paper, and getting a good negative saves time and effort in printing.
Keep in mind that color film has much less exposure latitude, and some black and white films have less tolerance. ADOX CHS 25, for example, has very little overexposure tolerance.
Without a meter, a exposure can be estimated using the f/16 rule. On a clear, sunny day, use f/16 as your aperture setting, and set the shutter speed to the reciprocal of the film speed. So, using 400 ASA/ISO film, the exposure would be f/16 @ 1/400 or 1/500. Less than bright sunny light, start opening up the aperture. Combined with bracketing, it works.