Here is the gerrit document's description regarding ACK:

Comments can be unresolved (something should be changed) or resolved (informational). If you have addressed an unresolved comment in a next patchset, you can quickly resolve the comment by clicking "Done" (if it was resolved in a next patchset) or "Ack" (if you acknowledge the comment, but don’t want to make changes).

https://gerrit-review.googlesource.com/Documentation/user-review-ui.html