PPP is a combination of several relatively sophisticated GNSS position refinement techniques that can be used with near-consumer-grade hardware to yield near-survey-grade results.
This is useful because a major source of position error is variability in how GNSS signals are slowed in the ionosphere, which is affected relatively unpredictably by space weather.
This process is only approximate, and non-dispersive sources of delay remain (notably from water vapor moving around in the troposphere), but it improves accuracy significantly.
More recently, projects such as APPS Archived 2021-04-21 at the Wayback Machine, the Automatic Precise Positioning Service of NASA JPL, have begun publishing improved ephemerides over the internet with very low latency.
This makes it relatively unappealing for applications such as fleet tracking, where centimeter-scale precision is generally not worth the extra complexity, and more useful in areas like robotics, where there may already be an assumption of onboard processing power and frequent data transfer.