Each major currency's average move against the other seven. Strong at the top, weak at the bottom.
For each currency we take its change against each of the other seven majors (for the euro: EUR/USD, EUR/GBP, EUR/JPY…) and average them, from the European Central Bank's daily rates. Pairing a strong currency with a weak one is a common way to look for trends; two currencies moving the same way tend to go sideways against each other.
It compares each major currency against all the others at once, so you can see which are rising and which are falling, instead of looking at one pair at a time.
Many look to buy a strong currency against a weak one, for example when the dollar is strongest and the yen weakest, USDJPY often trends up.
Throughout the trading day, for today, this week and this month.