When I right click from within the Microflow and go to properties, I think things are bit more clear there than looking in the properties window. I hope that could have resolved your confusion probably. 

It says clearly, disallow concurrent execution. Which is, by default, the Microflows can be executed by multiple users. 

For example, if I have a Login button → ofcourse this button must be executed by everyone, so by default we dont have to check the check box.

On the contrary, if there is a SYNC button or some MF that updates your master data or something and if you dont want to take a chance, you could just just enable this option and provide a user friendly message. A classic example from our case: we have a SE which updates something in a time period of 5 mins. Lets say, if within 5 mins if the SE is not finished, then a new instance of execution starts up which will pick up new set of items to be processed. So to avoid concurrent updates, we disabled concurrent execution. 

In principle, everything you model in Mendix executes sequentially. You can get concurrent executions by a user clicking a button twice, or two different users calling the same action at the same time.

To execute microflows concurrently from another microflow you will need to use the Java Runtime API (documented here) and you need to use the following method (documented here):
 

executeAsync​(IContext context, java.lang.String microflowName, boolean executeInTransaction, java.util.Map<java.lang.String,​java.lang.Object> params) 

This is what the properties of a microflow look like on my machine.

How do you execute it multiple times? Is it a button click? is it another microflow that executes this microflow in the background? How do you make sure that it is a concurrent execution?

If you set it to yes, concurrent execution is disallowed. If you want to allow it, set it to “No”.

Please, could youshare the error message with us that you get?

I had the same error massage.
What I needed to do was just enter an error massage in the Microflow properties:
in my example it was written like this


and I just removed the brackets.


the error massage is really confusing at the beginning and it is not clear from the beginning what the problem is.
Maybe Mendix need to think about a new Error massage that describes the error better.