A plectrum is a triangular piece of plastic (or wood) that we hold with our index finger and thumb to strum or pluck the strings.
As a beginner you should be using an extra thin pick. Here’s my recommendation… Tortex Red
Here’s another link for a mixed bag of picks at varying thicknesses… 12 Dunlop Tortex Guitar Picks
TOP TIP – In your first 90 days keep checking yourself to see if you’re holding the pick correctly.
Your index finger is on it’s side as you grip the pick.
This Video has even more detailed information on how to strum the strings effectively.