To make six times as much or as many.
Sextuple is a very big and impressive thing, like having six of something when you usually only have one.
Examples:
If you have one cookie and I give you five more, then you have sextupled your cookies!
She sextupled her money by winning the lottery.
The family sextupled in size when they adopted five more children.
I sextupled the number of flowers in my garden, and now it's very pretty.
He sextupled his speed and won the race by a lot.
The cake was sextuple the size of a normal cake, and it was delicious.