pullup resistance
Basically, you put pull-up or down resistors, to provide a stable voltage reference to inputs or outputs of a microcontroller.
It's pull up or pull down depending on the device itself, there are some devices where the inputs or outputs are designed to drain current, then you use pull-up, otherwise you use pull-down. The value of the resistances depends on the amount of current that the inputs or outputs can handle.
You can find all that information on the user manual of the device you are using, in general they give you for example
current in mA that each input or output can handle, and the voltage, for example 5 V, then easily you can find the pull-up or pull-down resistance by R = V/I.
If it's a communication bus like those you posted, the current is determined by a standard, to create kind of a current loop to feed several devices.