Leading away from an ace
The normal lead from a sequence of honours is the highest card in the suit. If you have a suit headed by an ace and you choose to lead a low card from that suit you're said to be 'leading away from an ace' or 'underleading an ace'.
South leads the ♠A, top of a sequence
On this next hand, the contract is 3NT. South is on lead but doesn't have a sequence of honours.
South leads a low card, the ♠3, 'underleading' the ♠A.
Underleading other honours
You might be familiar with the idea of leading low from an honour. Or fourth highest from your longest and strongest suit. If that suit includes an ace you're underleading it.
It's not just aces! Underleading other honours means the same thing.
South is on lead against 3NT again.
South leads a low card, the ♠4, 'underleading' the ♠K.
Underleading an Ace against a suit contract.
Any time you underlead an honour there's a risk that you might not end up making your honour at all. But often the best chance of defeating a contract is to set up tricks in your long suit so it's worth trying.
However, against a suit contract it's just not worth the risk to underlead an Ace.
The contract is 5♦ by East. If South leads a small spade West's ♠K will make and declarer will be able to ruff any further spade leads. That's bad for the defence.
Don't lead away from an ace in a suit contract.
Same hand but this time the contract is 3NT by East and South leads the ♠3. West's ♠K makes but as soon as the defence regain the lead they can take four spade tricks. Underleading an ace against notrumps is fine because you'll almost always still be able to take your trick later.
Underleading an ace against notrumps is fine