The UK lightning detector, explained:

CG stands for Cloud to Ground. IC stands for Intra Cloud. The + and - indicate the measured polarity of the stroke.
As the strokes age the initial symbol changes to a small triangle for CG and
a small dot for IC. The triangle points up for CG+ and down for CG-
The colour of this symbol varies from yellow to red as the strokes get
older, so a red marker is nearly 60 minutes old.