Steeping is the process of allowing the flavors to mix together chemically. The more complex flavors need more steeping time, simpler flavors less, but that's not a hard rule, each flavor is different.
The process itself is easy - Shake the bottle, uncap it (take the dripper off if it has one), and put it in a dark drawer or cabinet for a few days to a few weeks. I usually recap mine after a couple of days. You should shake them every day, but I forget.
Some of them will change color, which can tell you it's ready, some you can tell by it's smell, but usually it's trial and error if it's a new juice. I try most of mine right away, to have something to compare to.
Keep in mind too the transit time for it to get to you - some only require a day or so, so they may be steeped by the time they reach you. The more expensive juices will have their "birth" date on them, so you know how long they might need. 5 Pawns, for instance, is steeped for a month before they ship it to you, so it's good to go right away.