If you click on a RootsSearch link from a private profile you will be asked for your WikiTree login info, even if you are already logged-in to WikiTree. This is because the tool that connects WikiTree to RootsSearch is an independent application run on our separate "apps" server, apps.wikitree.com. It is safe to enter your WikiTree credentials here.
Many of the linked websites require a further login. For example, to search FamilySearch you need a FamilySearch login. Although FamilySearch is free, not all the linked websites are. Some require you to pay for a membership to see the information on their site.
You can change which websites have search buttons at the bottom of RootsSearch. Click the small "more sites" link at the bottom to change the settings.