It's party wealth. So the total of all items and gold combined between all characters at level 6 should total up to 2000g. But that's just a generalization. If you and your party have fun than you are doing well.
Edit: read the reply to this one for more clarification.
The 2000 gp listed at level 6 isn't a total for their entire career up to that point. It's the amount of treasure they are suggesting be handed out to the whole party between the time the party turns level 6 and the time they turn level 7.
That said, it's not at all far off from the amount that 4 brand new characters would get from table 10-10 Character Wealth if they were all being created at that level.
In my opinion, if the party as a whole is below that number, you should really be planning to hand out some extra rewards to try to catch them up. If they're between that number and double or so, you're probably fine. If they're more than, I don't know, like 5x that number, I would slow down on the loot for a while.
I think there are two tables of interest to get a feel for things. We have the Treasure by Level section, and the Treasure for New Characters section.
Looking at the Treasure by Level chart, we see that while the characters were level 5, they should have gotten about 1320 gp worth of treasure for the whole group. The suggested spread for that is a couple of level 5 items, a couple of level 6 items, a handful of consumables ranging from 4th to 6th level, plus about 320 gold. Over the course of an entire career running from 1st level to just turning 6th level, the party probably should have gotten about 3175 gp worth of stuff, split amongst the entire group. Some of the consumables probably should have been used over the course of those levels, and some magic items may well have been sold, so they should probably have a bit less than that left. Again, that's for the entire party. It sounds like your PCs have about 4000 gp worth of stuff split between 4 characters. That's a bit more than the chart calls for.
If we look at the Treasure for New Characters charts, we see that a brand new 6th level character would get either a handful of items ranging from 2nd to 5th level plus some gold, or else a lump sum of 450 gp. It's normal that characters that were played through the lower levels end up with more stuff than a character who just goes shopping at level six. The characters who played through those levels may well have some less-than-optimal items, just because they found them along the way, plus they get some advantages just from having been in play longer.
All of that says that your PCs sound like they're a little more wealthy than the guidelines, but not by a ton. If you wanted to, it would be fine to back off on the loot a bit over the course of level 6. On the other hand, I think you're basically pretty close. If you're comfortable with the power level of their gear, just keep on with what you've been doing and don't worry about it.
That's really helpful, thanks! We all just got out of a pretty stingy 5e campaign, so I'm ok with them being a bit rich. Just wanted to make sure I'm not completely out of bounds.
for reference, Paizo constantly breaks the wealth per level rules in published APs and even organized play, so I wouldn't sweat it too much unless this noticeably and negatively impacts your game