Plain English Meaning
A specific generation of GPS satellites that orbit the Earth and broadcast signals your GPS receiver uses to figure out where you are. The 'IIF' (two-F) just identifies which production batch they belong to.
Full Definition
The fourth generation of GPS satellites operated by the United States, launched between 2010 and 2016 to replace aging earlier-generation satellites in the GPS constellation. Block IIF satellites added a third civil signal (L5) intended for safety-of-life applications such as aviation, improved accuracy, longer design life (about 12 years), and more accurate onboard atomic clocks compared to earlier blocks.
Context Anchor
Seen in instrument flying material when the explains the space-based part of and how GPS capability has improved different generations.