Takbir is a very important phrase that Muslims say to show their love and respect to Allah, which is the Arabic word for God. It is used to start lots of Islamic acts of worship, like prayers, fasting, or festivals to celebrate important events in Islam.
When Muslims say "Allahu Akbar", they are saying that Allah is the greatest and most important thing or person in the whole world. It's a very special phrase that reminds Muslims to always remember Allah and to give him thanks and praise for everything in their lives.
So, just like when we tell our parents how much we love and appreciate them, Muslims use takbir to express their love and respect for Allah.