Acknowledgement: a document that acknowledges something.
The base: Acknowledge
An example for the second meaning is, "Please acknowledge me by allowing me to speak!"
The suffix: -ment
The whole thing: acknowledgement
To acknowledge is a verb, but adding the suffix -ment makes it a noun. Acknowledgement. If it were to be taken literally it would be "the thing that acknowledges".
The picture below is of an acknowledgement.
An acknowledgement of thanks would be used if someone did something important and they wanted to thank people who they think helped them along the way.