What does this mean ?

My first guess would be a dirty or malfunctioning battery fan.

 

Or just a clogged filter...

 

The HV battery uses forced-air cooling, and the air intake, on the front of the right rear seat, has a filter. In the Owner’s Manual (PDF) for model year 2017, see pages 573–574. (The 2016 editions mention the filter but lack the cleaning instructions.)

 

Probably filter and fan need cleaning. And the ductwork while you're in there.

Toyota could use a bit more common sense in their warnings. Won't be happening tho.

 

“At your dealer” was changed to “See owner’s manual” for model years 2017 and later.

Perhaps the designers didn’t anticipate frequent clogging of the intake with pet hair or other substances that could be removed effectively using a small cordless vacuum cleaner, as opposed to dust or dirt trapped in the filter itself, which requires disassembly to replace.

 

I should probably take a look at mine. Haven't even taken a look at it ever. Anybody know the part number of that filter?

 

It’s G92DH-47070 for all fourth-generation Prius cars. See my February 2018 posting and this previous thread with pictures.

(Prius Prime and Prius PHV use two filters, G92DH-47080 and G92DJ-47010.)