Pooling is something that yarn does, not something yarn is. It is usually undesirable, but sometimes planned.
If you have a yarn like this, the colours can form unwanted blobs, instead of spreading out in a nice way:
This is a massive problem with variegated or multi-coloured yarns, especially those gorgeous handpainted skeins you picked up at a wool festival for a small fortune, only to realise they look prettier in the skein than they ever will knitted up (~looks mournfully at the “special” shelf in her craft room~).
BUT: if the colour changes are regular, you can do a thing called ‘planned pooling’, which produces effects like this:
instead of this: