Read-only visitors (that find the desired information quickly)
Visiting the site and learning something new or re-learning something forgotten is surely meaningful engagement with the knowledge base content. It's especially meaningful if you find the things you are searching for quickly, so the pure number of visits may not be the optimal measure there. This is kind of passive engagement, but also one of if not the main activity of a knowledge library.
Active participation requires an account. It's meaningful as a prerequisite for active contribution.
Voting (especially on new contributions)
Is important to give feedback about the quality of a contribution and to separate the wheat from the chaff early on. It basically is an indicator of signal and noise, but beyond a certain threshold, it largely correlates with views. For example, questions about Git on SO score relatively high, but that doesn't mean they are great questions.
Asking (good, non-duplicate) questions
That basically requires a certain minimal question asking skill and often also a minimal understanding of the subject. It also requires thorough (re-)search and having thought about the subject for quite some time. If done right, there is a chance to get the problem solved and it's a necessary contribution to the knowledge Q&A. Somebody has to write these questions in the knowledge base.
If however, the question is not received well and ends up with a negative score or gets closed and especially if this happens too often it's a non-meaningful engagement. Something (expectations, guidance, lack of effort) has gone wrong.
Asking duplicates is somewhere in between, depending how much search effort has been invested, but in general finding duplicates is regarded a difficult problem still.
Answering (on-topic) questions
Is surely meaningful, but the meaning may be scaled with the gain that the asker and all future visitors get out of it. On SO there is the idea that many of the easy to answer and popular questions have already been asked and that's why the number of views per question and per time decreases. New questions are more specific and get seen less often. That may be a reason why the ratio of new answers to new questions has fallen on SO from approx. 2:1 to now below 1:1 over the last 10 years.
Commenting (on the content)
That's often the glue that is everywhere and improves the efficiency of the whole Q&A process. Comments on new questions are often the key to improving the questions, and highly upvoted comments on answers often indicate crucial information. Comments are often the first step to edits. The only drawback are the ~1-2% (YMMV) of unfriendly comments, they are not meaningful.
Whether it is removing superfluous verbiage ("Thank you", "Hope that helps"), correcting spelling, editing in of comments or just generally improving the content, many contributions have already benefited greatly from editing. Editing may even be underestimated still in its importance. The editors polish the content but do only get a minimal share of reputation for it.
One problem is that the higher the quality of the existing content, the more quality could get lost by a bad edit. Not all edits are good unfortunately, even if they were meant to be.
On the other hand, old content ages. Editing it, if only to indicate problems now, is a big improvement.
Moderation and curation (via review queues)
Reviewing is quality control and exception handling. Directing attention to new contributions or flagged contributions, it helps keeping the quality high and removing unsalvageable content.
Problem is that reviewing is hard work and the capacity for it is limited (and may even compete with other meaningful engagement). If people ask too many bad questions or post too many unfriendly comments, then the cleanup team has to work overtime. It may be more meaningful then to tackle the quality problems closer to where they are created.
It may be difficult to actually quantify and measure this meaningful engagement. It's not the sheer number of visits, questions, answers, votes, edits, comments or review actions, but it's more like a quality weighted number of these minus the number of meaningless visits, questions, answers, edits, comments, time needed for review.