It means up to 10 records in your address book with which you can actively work, including concurrent use.
By "actively working" here I mean that, in theory, you can add more than 10 records to your address book even with the free license, but in this case you won't be able to establish a remote connection. The program will ask you to remove extra records and reduce their number to 10.
My business model is that I need to be on 1 or 2 customer computers at most at the same time. However, over the span of a month, I may be on 30 different customer computers.
If you still want to stay within the free license limits, you will have to replace records in your address book with new ones so that their total number is <= 10.