In one line
CPC (Cost Per Click) is the cost of a single click — equal to CPM ÷ CTR.
Why it matters
At the same CPM, higher CTR means lower CPC. So when CPC rises, figure out whether the platform got more expensive (CPM up) or your creative stopped working (CTR down) — the fix is completely different depending on which it is.
Go deeper
What to do when CTR is low is covered in Improving CTR.