A pin's brightness shows how recently that location last listened, over the whole period shown at the top.
Bright, solid — heard just now, or very lately.
Faint, ghostly — not heard for a long time.
Everything in between scales smoothly. Click any pin for when it was first and last heard, per channel.
Why not how long? A stream stays connected even when the teletype beside it is switched off, so connection time does not measure listening — and it means something different on the always-on channels than it does on AUTOSTART.