Standard/concert pitch is the note we use to tune all instruments relative to. In Western music, this is A4 = 440Hz.
However, what this doesn't tell you is the relationship between frequencies of notes, which are called intervals. To work this out, we need a tuning system or temperament, which tells us the ratios of frequencies for every interval. For most instruments made nowadays, we use a tuning system known as equal temperament. This is a system where the octave is divided equally into 12 semitones, meaning the frequency ratio between each semitone is identical. We can represent the ratios between intervals using cents, where 1 cent = 1/100th of a semitone. This means that each semitone in equal temperament is 100 cents wide. And as a generality, any deviation from an equally tempered note we called a microtone. Equal temperament is a choice; it's how we've decided to tune our instruments. But, technically, it is out of tune. It's not the natural way instruments should be tuned.
The temperament that is most in tune is called just intonation, or pure intonation. This tuning system is based on the natural harmonic series of musical notes. If you play a note on an instrument, although you may think it's made from a single pitch, what's actually produced is a complex tone made up of several different pitches, called harmonics. These harmonics are formed by all of the sound waves combining and overlapping.
Jacob Collier is aware that pitch and tuning are not fixed, temperament, in particular, is something he takes advantage of a lot. His use of different temperaments can lead to some remarkable results. He uses tuning, pitch, in particular, to add colour to his songs.
Jacob's song, 'Hide Away", is a good example. The track starts in A4=432Hz, but shift's to A4=440Hz by the end of the first verse. The way he shifts pitch is incredibly subtle. He uses the second note in the melody, which is the major 3rd in D major, to pivot between A4=432Hz and A4=440Hz. By tuning the major third closer to equal temperament than just intonation, when it eventually shifts into the higher pitch the same F# ends up being justly tuned in A4=440Hz.
The last verse of 'In The Bleak Midwinter' is in a microtonal key that's halfway between G and G# Major (G 1/4 # major).