PULL UP RESISTORS usually a 4.7 k resistor connected to VCC to pullup the signal line to VCC when not being held low by a closed end stop switch.
Digital lines have two states High or low, zero volts or ~+Vcc volts or simply stated zero or one, ('on or off').
In the Marlin firmware ENDSTOPPULLUPS is defined by default, and ENDSTOPPULLUPS for each end stop connector on the motherboard are enabled individually.
Some end stop switches built on printed circuit boards already have the pullup resistors included like the ones I use for my homemade filament run-out sensor switch.
You may have mechanical end stop switches for axis X and Y that need pull up resistors enabled and optical end stops that don’t need pull up resistors enabled.
Digital lines have two states High or low, zero volts or ~+Vcc volts or simply stated zero or one, ('on or off').
In the Marlin firmware ENDSTOPPULLUPS is defined by default, and ENDSTOPPULLUPS for each end stop connector on the motherboard are enabled individually.
Some end stop switches built on printed circuit boards already have the pullup resistors included like the ones I use for my homemade filament run-out sensor switch.
You may have mechanical end stop switches for axis X and Y that need pull up resistors enabled and optical end stops that don’t need pull up resistors enabled.