What is a sencer?

If you mean sensor, a sensor is a device that changes based on some phenomenon and this change is used to measure the change in the phenomenon.

For example, assume a device produces a voltage and the amount of that voltage depends on the temperature of the device. It can be used as a sensor for temperature. Assume you put the device in water and another device measures the voltage produced by the sensor. Based on the voltage of the sensor it can display the temperature of the water. So, in this example, the sensor senses the temperature of the water.