Insulators vs. Conductors
Electricity is conducted through some things better than others. Its resistance measures how well something conducts electricity. Some things hold their electrons very tightly. Electrons do not move through them very well. These things are called insulators. Rubber, plastic, cloth, glass and dry air are good insulators and have very high resistance.
Other materials have some loosely held electrons, which allows movement through them very easily. These are called conductors. Most metals - like copper, aluminum or steel - are good conductors.