Sorry if this is a newbie question, however on some leads I see "Converted Account: Name of lead’s account) on the top of the lead detail view:
I was not able to figure out, when an account is considered “converted”?
Any input is highly appreciated! Thank you!
OK … you are a newbie 
A converted lead is part of the sales process.
Consider … someone completes a form on your website which then creates a lead in SuiteCRM.
At this point you have no idea if that person in truly interested in your product/service or not.
Usually, a salesperson will contact the Lead and qualify them.
If they are a genuine prospect for business, then they will “Convert” the Lead (there is a button on Lead DetailView for this purpose).
One a Lead is converted, then you will see the notification that you posted about.
Thank you very much for your reply! That much I know about the sales process 
What is confusing me however, is that the leads I’ve mentioned are not converted yet. They have the status “new” and yet the conversion notification in the header is shown. Am I missing something here? Any pointers on what I could check?
Thank you for your appreciated help.
If you click on the Acount Name in the “Converted Account” field … does it take you to the account? If so then the relationship between Account and Lead is formed … so it has been converted. A lead can be converted even though the status is new if the Lead has been edited after conversion and status set to new (during conversion the status is automatically set to "Converted). Setting it to New does not affect the Lead>Account relationship.
Thank you, I’ll look into this. This concerns another person’s CRM account, most likely they have converted the lead for some reason in the past.