The electrical resistance of an object is a measure of its opposition to the flow of electric current. Materials like plastic, rubber etc. offers high resistance and metals generally have low resistance. Resistance depend on the shape and size also. A longer wire has more resistance than a shorter one. A thicker wire has less resistance than a thinner one.
There are different types of resistors available commercially. Refer to Wikipedia for more details.
The screenshot of a measurement on the Android version is shown below. It should be noted that the range of these measurement is rather limited. Internally, the terminal SEN is connected to 3.3 volts via a 5.1 kOhm resistor. Voltage at SEN is measured and the resistance from SEN to Ground is calculated using Ohm’s law. The measurements are accurate when the connected resistance is comparable to 5.1 kOhm.
In the PC version the oscilloscope screen always display the value of resistance connected between SEN and Ground.