It's folder to save part of your blockchain - ancient database for ancient serments(contains away past blocks and receipts). By default it has created in geth/chaindata but you can transfer it in another place using parameter in command line.
It is used by reorg chain or redownloading. The use of this database is called "freezer". You can read ...
You might try popping this in the geth console, tweaking the block production rate for your chain of interest:
const secPerSample = 10;
const sampleWindow = 100;
const networkBlocksPerSec = 1 / 2.3; // network block time (polygon makes a block every ~2 seconds)
const dataPoints = ;
var topBlock = eth.syncing....