Motor Speed Output Signal
OK, the details are thin but I'll take a stab at answering.

RPM or tachometer signals are frequently generated by Hall Effect sensors. The resulting signal will be a square wave whose frequency is proportional to the rotational velocity.  Some motors have a tach signal that generates more than one pulse per revolution.  You'll have to experiment to determine the number of pulses per revolution or read the data sheet if you have one.

A common way to measure the RPM is to connect the tach signal to a timer/counter in your microcontroller and count the time between similar edges. For lower RPM ranges another way is to use interrupts. For each tach pulse edge (rising to rising or falling to falling) generate an interrupt and note the value of a free-running counter. Do the math and voila, RPM.

You will frequently need a pullup resistor (10K or so to VCC) as the Hall Effect sensors will typically only drive low. Read the datasheet if you can find one.