An emphasized Sunnah is a practice observed with particular insistence. Calling the adhan for regular prayer is among the public symbols of Islam.
Adhan itself is not a condition for the validity of prayer: an obligatory prayer is not nullified solely because adhan was omitted, although omission of a public Sunnah has a separate legal ruling.
Some jurists discuss its strength in terms of obligation; the present text expressly characterizes it as Sunnah.