@laurel_dog Local and Federated timeline will contain "Public" posts, if you don't want your messages showing up there I recommend posting as "unlisted" or "followers only".
Federated timeline are all connected instances, both Mastodon and other softwares.
Local timeline is the instance you are on (mastodon.social in this case).
Follow friends and discover new ones. Publish anything you want: links, pictures, text, video. This server is run by the main developers of the Mastodon project. Everyone is welcome as long as you follow our code of conduct!