It's essentially the same as the "AMOLED" display on the regular Pixel 2.
The Pixel 2 has an "AMOLED" display, which Samsung's Galaxy smartphones have featured ever since the first Galaxy model was released back in 2009. AMOLED displays are essentially the same thing as pOLED displays, as they also have a plastic base layer instead of glass. It's not clear, however, if the Pixel 2's AMOLED display is manufactured by Samsung.
pOLED panels can be thinner, lighter, and cost less to manufacture than regular glass OLED panels, which makes sense for mobile devices.
Depending on what type of plastic is used, pOLED displays have the potential to be flexible, or even foldable. It could lead to interesting design choices in the future that glass OLED panels wouldn't cater to.
And because OLED-type displays turn off entirely when displaying the color black, it allows for the "always-on" mode on smartphones, where a phone can display the time and other basic information on the lock screen without using up much battery.
So what is the "p" in front of "OLED" in the Pixel 2 XL?
The "p" stands for plastic. pOLED is a display made by LG, the company that manufactures the Pixel 2XL for Google, and it uses a plastic base layer instead of glass like most OLED panels, according to Android Authority. pOLED is also found in LG's most recent V30 smartphone.
"pOLED" is part of the "OLED" display family.
OLED panels are mostly found in some high-end TVs, several Android smartphones, and most recently in the Apple iPhone X.
In general, OLED panels are better than LCD (liquid crystal display) panels in almost every way. They offer incredibly rich color reproduction compared to LCD panels you'd find in older Android phones and the iPhone 8, as well as better contrast between bright and dark areas.
OLED panels can also reproduce a perfect black color because the pixels displaying the color black will actually turn off entirely. When an LCD panel displays the color black, it tends to appear more gray because the entire LCD panel is constantly backlit, even when there's the color black being displayed. The liquid crystals in front of an LCD panel's backlight can't block out the backlight entirely, and thus the color black can rarely ever be properly reproduced.
By allowing pixels to turn off entirely for the color black, OLED panels can improve the battery life of mobile devices, too, especially when viewing darker content.
