The lead of an honour card. Unless it is from a short suit, the lead of an honour in standard methods promises the honour immediately below it.
For example, the lead of the jack would be consistent with holdings of J-10-9-x, J-10-8-x-x, K-J-10-x-x or J-10-x. Against a suit contract, you would also lead the jack from J-10-x-x. Against a no-trump contract, you would also lead the jack from A-J-10-x-x.