There are several fundamental concerns and techniques used in recording sound on any medium.
Noise floor: Every recording medium adds noise into the recording. In purely theoretical terms, digital recording does not, but rather the analog equipment (the mic, cables, etc.) connected to the digital equipment contributes noise. We can listen to a recording of silence in various mediums and listen to the noise floor. How do we get rid of this noise? Yes, we can attenuate the signal. But what if there is some recorded material that we now cannot hear?
Signal-to-noise: When we talk about a recording (or an amplification setup like a P.A.) we are talking about an electronic signal that must compete with the inherent noise for our ears' attention. The more signal and the less noise, the more we can understand whatever it is that we are listening to. Signal-to-noise is a term used by equipment manufacturers to tell users how good their equipment is at delivering a clear sound. In the recording studio, it is a term that is used to tell us how good our recording is in an objective way. The cleaner the signal, the more freedom you have to boost the signal without hearing unwanted noise. The noisier the signal, the more you will have to attenuate it when mixing with other sounds.
Levels: When two or more sounds are mixed together, they must be mixed at levels such that both can be heard in a relationship to each other. They do not necessarily have to be at the same level; your ear and your objectives will guide you to the right levels. This brings us to one rule of thumb: You can always attenuate a sound; you cannot always amplify a sound. If you record a sound at too low of a level, you will introduce noise into the recording later that you cannot remove without rerecording the sound at a higher level.
Filtering: Sounds are constantly being filtered. Our ears act as a type of filter. Our mouths filter sounds produced in the throat. Our electronics filter any sounds that pass through them. Another rule: Filters are never perfect. The perfect theoretical filter would act as a brick wall: removing all unwanted frequencies and passing all wanted frequencies. This never happens in the real world. Every filter exhibits roll-off, or a band of frequencies where the sound is partially attenuated.
Stereo imaging: Most of us have 2 working ears that allow us to locate sounds in space. This is why most stereos have 2 speakers instead of 1. (This was also good when we had to hunt for our dinner.) In a studio we can take advantage of this physiological trait by mixing a sound between 2 or more speakers. We will talk about soniclocutioin (the process of locating a sound in space) in more depth as we learn to use ProTools.