Every electrical device you have used has something called electrical resistance. If you've noticed electrical devices get warm - perhaps a calculator or cell phone or a stereo or computer that's been on - that heat is the sign of electrical resistance.
When electric current flows through anything, where the current is a flow of electrons, those electrons collide with atoms within the lattice of the material. Even good conductors like copper wire and gold, will get warm...there are no perfect materials, and atoms get in the way of the flowing electrons!
This video will use a simulation to get some images in our heads as to what happens at the micro-level. And also remember this: in mechanics, all collisions have some loss of kinetic energy. So every electron loses KE in a collision to that atom, and the atom then vibrates in place a little faster. From chemistry, we know that the temperature of a material depends on the average KE of its atoms and molecules - so if there is an increase in vibrational KE of the atoms of a material, it feels warmer to us!
Materials that create few collisions for the electrons in the current have low resistance (small numbers of ohms), and if there are a lot of collisions there is a higher resistance (larger number of ohms, the unit of resistance).
See another video for how we handle multiple resistors in circuits, in combinations of series and parallel, that we need for circuit analysis.
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