Hi Bianca, 

I think it's best explained here: https://help.totaralearning.com/display/TL12/Audiences (search: "Visible learning"). 

In essence, it is an option available in Site administration - Advanced features which when enabled, gives you the possibility to set visibility level for courses (which translates to access in this case) based on audience. 

You can find more info also here: https://help.totaralearning.com/display/TL12/Advanced+Features#AdvancedFeatures-Audiencebasedvisibility

Hope this will help. 

Best, 

Vlad