Interdigitation just means that the devices (transistors, resistors, capacitors, diodes) are distributed in an interleaved maner. For instance imagine that you must match two devices (A and B) composed by 4 unit devices each:
ABABABAB
---------> If there is a gradient in this direction component B is more affected than component A
An interdigitised structure would be
ABAB|BABA
Notice that in this case there is an axis of symmetry (I added character | for the axis) so any "linear" gradient is compensated. If now A and B are divided into 16 unit devices, a common centroid structure would be
ABAB|BABA
BABA|ABAB
--------------
BABA|ABAB
ABAB|BABA
Notice in this case that the structure has vertical and horizontal symmetry axes, or more general central symmetry.