Top of a Sequence Leads
When you have a sequence of touching honours leading the top of that sequence let's partner know about your holding in the suit. The lead of an honour shows the honour below but denies the honour above. Without a sequence you'll often lead your fourth highest.
On the following hand South leads the ♠Q. North now knows that South has the ♠J but not the ♠K
The next two hands look similar but the opening lead of top of a sequence gives South the right information to defeat the contract each time.
North leads the ♠K against West's 3NT contract. Upon winning the ♦A, South returns a spade, knowing that North has the top spades.
South's hand and dummy are the same on this next hand but the opening lead is different.
This time North leads the ♠Q against West's 3NT contract. If South returns a spade, declarer will win that trick and play all the diamonds. South's only chance to defeat the contract is to switch to a heart and hope North has the ♥A.