Display prints "Z Endstop" but is still printing?

What does it mean when the Marlin F/W prints Z-Endstop on the display during a print?
The print keeps going and nothing special seems to happen.
What does this mean?

Thanks,
DLC

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Kits: Folgertech Kossel 2020 upgraded E3Dv6, Anet A8 upgraded E3Dv6, Tevo Tarantula enhanced parts and dual-head, TronXY X5SA Pro(E3DHemera).
Scratch: Large bed Cartesian, exchangeable heads, Linear slide Delta, Maker-Beam XL Micro Delta, 220x220CoreXY.

My MBot clone does that when the endstop switch is triggered. Since it uses one as a sort of Z probe, this sometimes happens when the print gets a little fuzzy. it does not stop it from printing or moving though.

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MBot3D Printer
MakerBot clone Kit from Amazon
Added heated bed.

Leadscrew self-built printer (in progress)
Duet Wifi, Precision Piezo parts