What is the Preserved Tablet in Islam?

Where the name comes from

The phrase appears at the end of Surah al-Buruj, describing the Quran itself:

"Lawh" means a tablet or slate, and "mahfuz" means guarded or protected. Nothing in it can be changed, lost or reached by devils. Elsewhere the Quran calls it "the Mother of the Book" (Umm al-Kitab) and "a clear register" (Kitab Mubin).

What is written in it?

Scholars explain that the Tablet holds Allah's complete decree: every birth, death, provision, event and choice, down to a leaf falling from a tree. The Prophet ﷺ said that Allah wrote the measures of all creation fifty thousand years before He created the heavens and the earth (Sahih Muslim 2653). He also said that the first thing Allah created was the Pen, and He told it to write what would happen until the Last Hour (Sunan Abi Dawud 4700).

The Tablet and the Quran

The Quran was written in the Preserved Tablet first. According to a well-known explanation from Ibn Abbas, it was sent down as a whole to the lowest heaven on Laylat al-Qadr, and then revealed to the Prophet ﷺ in parts over about 23 years. See What is Laylat al-Qadr and how do I find it? This is one reason Muslims see the Quran as Allah's own speech, not a human book. See Who wrote the Quran?

Does it mean I have no choice?

No. The Tablet records what Allah knows you will choose; it doesn't push you. Some scholars also mention that the records held by the angels can change through dua and good deeds (Quran 13:39 speaks of Allah erasing and confirming what He wills), while the Mother of the Book itself never changes, because Allah already knew the final outcome. You still act, pray and try your best.