This summary from Elizabeth Mackie explains the principle of subsidiarity.
Prepared for the Peace & Justice programme of CCANZ.
This supremely important principle of social philosophy, one which cannot be set aside or altered, remains firm and unshaken: Just as it is wrong to withdraw from the individual and commit to the community at large what private enterprise and endeavour can accomplish, so it is likewise unjust and a gravely harmful disturbance of right order to turn over to a greater society of higher rank functions and services which can be performed by lesser bodies on a lower plane. For a social undertaking of any sort, by its very nature, ought to aid the members of the body social, but never to destroy and absorb them.
Pius ZI, Quadragesimo Anno, 1931