Sensored systems use sensors in the motor that tell the esc where the rotor is at any given instance. Sensored motors have an exctra bundle of wires that plug into the speed control in addition to the three power wires.

Sensorless systems rely on back emf from the motor to determine where the rotor is. When the motor isn't moving, there is no way for the esc to know exactly where the rotor is. The speed control goes through a quick "start up" algorithm that gets the motor going and once it is able to detect the rotor position it functions very similarly to a sensored system.

Since you posted "question about TEKIN esc", I'm guessing you are debating between the R1 and the RS. The only difference between the two is that the RS can use the sensors on sensored motors where the R1 only uses the sensorless technique. The RS can run sensorless motors as well. If you plan to race, the only way to go is sensored since the starts are more consistent. If you just wish to bash and fear the extra sensor wires getting ripped off, go sensorless.

Unless you find an R1 for CHEAP, there is no reason to get it because the RS can do all the R1 can and more. I have 3 RS esc's and absolutely love them.