A Source allows you to connect to a social media provider to fetch Posts from. Sources can then be combined in Feeds as a content aggregator.
Each provider will be different, but almost all require OAuth authentication. Create a Source and follow the documentation for the provider to get your Client ID/Secret credentials. Once configured, connect to the provider, going through the OAuth handshake to retrieve a token.
Once connected, you'll be able to define what content you want to fetch from the provider. For example, Facebook can fetch Posts from a public Facebook Page, or just Videos. For Twitter, you can fetch Posts from a user, or from a nominated hashtag.
To fetch the Posts for a source, use source.getPosts().
{# Get the source by its handle #}
{% set source = craft.socialFeeds.getSourceByHandle('mySourceHandle') %}
{% for post in source.getPosts() %}
ID: {{ post.id }}<br>
Content: {{ post.getContent() }}
{% endfor %}
Check out our guide on Rendering Posts for more.