Well.... the most descriptive is the one in documentation, under the link I provided you. You can as well search the community. But basically its really simple. You can even put values and then run the workflow to see if they work for you 
I don't think this http://help.nintex.com/en-US/nintex2013/help/#Workflow/RootCategory/Actions/Nintex.Workflow.QueryUse... description requires really more assist By user you just put user login or email (so something using what SharePoint people picker field can find the proper user object), then you just adds properties from the matching user profiles you want to get back in return. You are looking for "Manager" property.
In advanced you can (but mustn't ) provide credentials of the account that has rights access user profile data, but as long as this is available to everyone (the "manager" property is), even a regular user with "Read" only permissions should be able to execute the action.
Regards,
Tomasz
Unfortunately in the current version I see the following, which means that it's not only in the advanced part. So if I understand it right I should provide the people's username and password to be able to do this action... If not please let me know how
That makes it clearer now You posted your question in the wrong forum, for on premise solutions, not O365.
For that purpose please visit this documentation: Office 365 Query User Profile
You have to provide e-mail address of the user whose profile you want to query. Then email and password of the user who has permissions to query the profile (there is no service account, you must provide credentials manually). Then you have to type in the property. In this situation it is simply: Manager
Regards,
Tomasz
I'm really sorry I was careless... And thank you
One last question: If I give my credentials, will the system provide datas every time or only when I'm initiating the workflow?
No problem, you welcome. I moved it to the right place
What do you mean by "providing the data"? When will it query user profile? It will do it once the action is executed. So you can put logic before, then execute the "query" action, and then use the action's outcome in another actions. It's a sequence.
Regards,
Tomasz
I just wanted to explore if only the person whose credentials are given can perform the action or not. But I guess no. I'm not in my top form today 
Yes, the "office 365" actions are always executed in person of the user whose credentials are provided. Even if you put it inside the "App Step" still it will be that "provided" user
Regards,
Tomasz
P.S.
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