So, a manifesto is like a big letter that a group of people write to say what they believe and what they want to do.
The Manifesto of the 121 was a letter written by a group of French people who disagreed with how their government was treating a specific group of people called Algerians.
You see, back then in France, Algeria was actually a part of France and some Algerians were living in France. But the French government wasn't treating them very nicely. They were being discriminated against and some were even being sent back to Algeria, even if they didn't want to go.
So, the group of people who wrote the manifesto wanted to say that they disagreed with the government and they wanted things to change. They wanted Algerians to be treated better and to have more rights.
Writing the manifesto was a big deal because back then, it was considered a crime to speak out against the government. But these people thought it was important to speak up and stand up for what they believed was right.