EV3 motor TACHO signal connection. Hardware schematic questions
The EV3 detects when motors are connected and disconnected using the ADC (and gpios). The two 100k resistors are basically a voltage divider, so when the motor is disconnected, the ADC will read 2.5V. If a motor is connected, the motor encoder will either pull the signal high (above 3.7V) or low (between 0.1 and 0.65V). The detection algorithm is actually a bit more involved than this, but this is the general idea.

For the full details, see the Device Connection Manager Driver and the actual source code. The documentation is not quite exactly correct, so when in doubt, trust the actual source code and not the comments (documentation is generated from comments).

And even though you didn't ask about it, I should point out that the transistor (Q10A) is also used as part of the detection.
You don't show it, but MA1 is connected to pin 2 on the output port, which is one of the motor driver signals, so MA1 could either be battery voltage, floating or ground depending on the state of the motor driver chip (via 1OUTB2). Additionally, the voltage could be steady or PWM depending on the speed and direction the motor is running. Or, MA1 could also be something else if there is no motor plugged in (or something besides a motor is plugged in).

On NXT sensors, pin 2 is connected to ground (pin 3). In the source code, it looks like this is just used to detect if a NXT sensor is plugged into the output port.