Lead visibility is when someone is either showing in the leads index or not showing in the leads index. One reason you might not want someone showing in the leads index is because maybe they are now a past client. So you just want them showing in the client section.
You can turn off someone showing in the leads index two ways. One is going to be from the 'All People' section. Once you are in 'All People' you will see an eyeball icon and if it is blue that means they are showing in the leads index. If they are gray that means that they are not showing in the leads index. All you have to do is click on the eyeball to change the color.
The second way to change the lead visibility is in their profile. Click into their profile and then click edit details. There you will see a section that says show in leads index, if it is checked they are showing in the leads index if it is not they are not showing.
You can also mass update lead visibility. You will want to go to all people and select the people that you would like to update. You will then want to click the action button that pops up and then select lead visibility. A window will then pop up and you can pick if you would like them to all show in the leads index or not show in leads index.