What is the meaning of -ieren?

Verb suffix: it and its inflected forms are found mainly as a rendering of French, Italian, and Latin suffixes like -ire and -iare in borrowings, but it is also sometimes applied with a factitive meaning to German stems.

used in verbs, indicating the action of doing something related to the first part of the word

third-person plural future subjunctive of -er

third-person plural future subjunctive of -er

Source: wiktionary.org