Compliance refers to the feel, the flexibility, of the string under the fingers (or pick) and is often confused with tension.
Tension is fixed for the whole string between bridge saddle and nut - given (1) fixed mass per unit length, (2) fixed scale length, and (3) fixed pitch.
But, although tension remains and must remain the same all along the string length, a string is more compliant when played at or around the 12th fret than it is when played near to the bridge.
Like tension, but not in the same way, compliance is affected by internal string construction - e.g. hexcore/roundcore - but possibly by other factors that have no effect on tension such as break angle at the nut.
Read all about it here ... http://www.liutaiomottola.com/myth/perception.htm ... and here ... https://www.liutaiomottola.com/formulae/tension.htm