It's a HDD cache memory chip, essentially. It's like an SSD, but made out of Phase Change Memory, so it's even faster and lower-latency than NAND used in SSDs. But it's very expensive. It's like $40-50 for a mere 16GB size.
Since the mobo in that PC has a PCI-E M.2 socket, you can also opt to buy the non-Optane model, with just the HDD, and then buy a separate Samsung 970 EVO M.2 PCI-E NVMe SSD, I recommend 512GB or larger, and use that as your primary bootable drive, and your video-editing scratch disk. Should speed up operations nicely. Yes, add that additional 8GB stick of DDR4-2667 RAM too.
I mean, I added up the parts cost to build that PC, and you can come in around $50 difference building yourself, and you'll probably end up with a standard higher-wattage PSU, more expansion options, etc., but you probably won't get USB 3.1 Gen2 10Gbit/sec ports on the front of your case. (Not aware of any cases sold at retail that have these, still pretty specialized.) Those are a pretty hot feature, IMHO.
I mean, that PC isn't ALL THAT, it's pretty average, but it's got the newer kit inside, Coffee Lake 6-core CPU, stock with 2667 RAM (rare to find above 2133 in pre-builts that aren't gaming rigs), and it's got the aforementioned USB 3.1 Gen2 ports on the front. Acer consumer laptops suck, but I've had good experiences with Acer and Gateway desktop PCs (same parent company these days). And it has a fair price.
(I was actually thinking about picking one up myself.)