Resistance is a measure of how much voltage an electrical element needs in order to increase the electrical current. The actual resistance depends on both the resistivity and the geometry of the wire. Resistivity is a property of the material (metals have low resistivity; wood has high resistivity). Resistivity of a material helps determine how well it will conduct electricity. A higher resistance results in a lower current for a given voltage. Resistance is measured in Ohms (Ω).
While it is easy to think of resistance as a bad thing, many electronic circuits require resistors (objects with high resistance, deliberately put in a circuit) to function! The resistance of a wire increases as its length increases and decreases as its thickness increases. When conducting electricity over long distances, like the electrical grid, it's important to keep the resistance as low as possible. The wires in the electrical grid have a low resistivity and fairly large cross-sectional areas to allow them to conduct electricity over long distances with minimal loss. Please play with the simulation below to see how these values affect resistance.
The University of Colorado has graciously allowed us to use the following Phet simulation. Explore the simulation to see how resistance changes depending on geometry and resistivity: