A microcontroller is a little computer that tells the electrons or electricity where to go and what to do or when to stop and when to go.
For example, like a police officer conducting traffic the microcontroller tells the electricity when to turn on and when to turn off. This creates LEDs that twinkle, fade, and/or blink.
When you look closely at a LilyTiny you can notice a positive tab (+), a negative tab (-), and tabs numbered 0-3. The numbered tabs are all positive (+)
The LilyTiny is preprogrammed to make LEDs do certain things. (Reference your "Design Your Circuit" page) For example tab 0=Fade, tab 1=Heartbeat, tab 2=Blink, and tab 3=Twinkle.
Even though the battery holder is different then the one we are using it is set up the same with a positive tab and a negative tab.
As you sew remember to NEVER cross your conductive thread traces. This is a BIG NO NO.
Same rules apply as the simple circuit-3 contact points with the conductive thread and positive (+) tabs sew to positive (+) tabs. Then knot and cut your conductive thread.
Your battery holder is only connected to your microcontroller (LilyTiny). Positive (+) to positive (+) and negative (-) to negative (-)