Can someone explain me what is the pool difficulty?

I understand what is the network difficulty, but the pool one got me off-guard.

From what I've read, this is what it means:

"Pool difficulty is how hard the job the pool gives you to finish in a time frame (set by pool)"

But I though the network difficulty defined the job for every miner (which is to pick a valid block).

What is a job then? I though that a job was basically the pool telling the miner: "here's a set of transactions, and here's your Nonce range [10000 to 99999], now check if you can hash below value X". Which I assume would have the same difficulty for every miner.

What am I missing here?