Antennas allow you to create secondary timeline of accounts, terms and tags without them having to appear in the primary timelines.

That way you can follow content from people without follow people.

Here the official explanation from Misskey:

Antenna is a feature that allows you to freely set conditions for a custom timeline and automatically collect matching notes. Antenna conditions can include conditions to include/exclude certain keywords and tags in different combinations as well as other options. When a note matching an antenna’s conditions is posted, the note will automatically be added to that antenna’s timeline.