Taqṣīr (deficiency) is to diminish someone's established rank and place them beneath the station rightly due to them. Just as exaggeration is wrong, denying a rightful excellence is also wrong.
An example of deficiency concerning the Messenger ﷺ is to place a prophet at the rank of an ordinary saint, or even below that rank. Prophethood and sainthood are not equal; the rank of prophethood exceeds that of all the saints of the community.
Recognizing rank correctly avoids both unjustified elevation and the denial of established superiority.