A small X symbol on a pin telling KiCad "I'm intentionally leaving this disconnected." Used for pins the chip exposes but your design doesn't need, so ERC stops warning about them.
How it looks
Real image: the CORE3588 schematic, exported by KiCad (shown in greys)
No-connect markers on the CORE3588 schematic: the small X shapes on unused pins. The schematic has 59 of them.
1No-connect X marks on unused pins 11 and 12 of camera socket P1.
2An X on pin 4 (NC) of the power switch U4.
How it works
Chips and connectors often have pins a design does not need.
Left alone, ERC warns about each one: "pin not connected", in case it was forgotten.
An X marker says "unconnected on purpose". ERC stays quiet, and anyone reading the schematic knows it was a decision.
Where it fits
Part of keeping ERC clean. On the layout, such pins get no track.