Azerbaijanis, who are close to the Turks in ethnicity, are the biggest minority of Iran, and this is the meaning of Erdogan.
While just a year ago, it had seemed that Turkey and Iran were getting closer together, this TV interview (in Turkish) teaches us that this may be a big mistake.
Erdoğan President of Turkey: "Iran wants to revive the Persian Empire in the region and expanded its influence throughout the region, but I tell the Iranians to wait, because inside Iran there are Azeri who will stop you."
The Azerbaijanis in Iran comprise the largest minority ethnic group in Iran. Apart from Iranian Azerbaijan (provinces of West Azerbaijan, East Azerbaijan, Ardabil and Zanjan) but Azerbaijani populations are found in large numbers other provinces, including central Iran, including many in Tehran itself.