Aged water simply means that the water has had time to mix with the salt.
I let my water age 24 hours before adding it to the tank. I use a 77L garbage container with a lid and put a powerhead for aeration and mixing.
Some people don't give it any time to age at all. Some people wait a few hours, some wait overnight, and some wait 48 hours.
Most salt brands brag of their product mixing instantly, like Instant Ocean as the name implies. Don't listen to that. There's several processes that take place during aging that is important.