Takbeerat al-ihram is one of the obligatory and first acts of the prayer; it is the declaration of Allahu Akbar (God is the greatest) at the beginning of the salah.

Why Allahu Akbar? Why not Alhamdulilah (all praise and thanks is to God), or la ilaha ila Allah (there is no deity but Allah)? Because Allahu Akbar means that Allah is bigger and greater and more important than whatever you are thinking of in terms of dunya (worldy) matters.