The takbeerat-ul-iḥraam refers to the first time that the praying person says takbeer ('Allahu Akbar'), raising their hands to between shoulder and ear level, with palms facing outwards, in order to begin the prayer.
The word iḥraam is a derivative of the Arabic word for 'forbidden', referring to the fact that many things that were permissible prior to this action (such as talking, eating, and drinking), become forbidden for the duration of the prayer.
The takbeerat-ul-iḥraam is another pillar of the prayer, without which the entire prayer is invalid.
The prayer is started by the takbeerat-ul-iḥraam; once said, you have entered the prayer and must fulfill the conditions of the prayer.
'Ali ibn Abī Ṭālib narrated that the Prophet (may the peace and blessings of Allah be upon him) said:
"The key to prayer is purifying oneself (wuḍoo'), it is entered into by takbeer (saying Allahu Akbar) and exited by tasleem (saying Assalāmu ‛alaykum wa Rahmatullāh)" [Abū Dāwood: 61; at-Tirmidhī: 3; Ibn Mājah: 275]