Drop C tuning is technically an alternative guitar tuning where at least one string has been lowered to a C, but most commonly refers to CGCFAD, which can be described as D tuning with a 6th string dropped to C, or drop D tuning transposed down a whole step. Because of its heavier tone, it is most commonly used in rock and heavy metal music.
In music, there are two common meanings for tuning:
Tuning is the process of adjusting the pitch of one or many tones from musical instruments to establish typical intervals between these tones. Tuning is usually based on a fixed reference, such as A = 440 Hz. Out of tune refers to a pitch/tone that is either too high (sharp) or too low (flat) in relation to a given reference pitch. While an instrument might be in tune relative to its own range of notes, it may not be considered 'in tune' if it does not match A = 440 Hz (or whatever reference pitch one might be using). Some instruments become 'out of tune' with damage or time and must be readjusted or repaired.
Different methods of sound production require different methods of adjustment:
C tuning is a type of guitar tuning. The strings of the guitar are tuned two whole steps lower than standard tuning. The resulting notes can be described most commonly as C-F-A♯-D♯-G-C or C-F-B♭-E♭-G-C.
The tuning is commonly used by metal and hard rock artists to achieve a heavier, deeper sound. It also makes bending easier for the player than in E standard using the same gauge of strings. Heavier gauge strings can be used in order to maintain tension in the strings. It is possible to play without them, but difficulties can be experienced as it becomes quite easy to unintentionally bend notes and chords out of tune. However, the ease of bends can be desirable, depending on playing style of the musician.
D♭ tuning, also called C♯ tuning, is an alternative guitar tuning. Each string is one and one half steps lower than in standard tuning, or one half step lower than D tuning. The resulting notes can be described as D♭-G♭-B-E-A♭-D♭ or (less often) as C♯-F♯-B-E-G♯-C♯. "Extremely slack," it is very popular amongst alternative and heavy rock bands because it has a darker and lower-pitched sound compared to E standard.