Electrical resistance is how much a material or component opposes the flow of electric current. High resistance makes it harder for current to flow; low resistance makes it easier.
Resistance and ohms
Resistance is measured in ohms. The site also has a dedicated explanation of what an ohm is, which is the unit used for resistance.
Why resistance matters
Resistance controls current, creates voltage drops, limits LED current, produces heat in heaters, and helps shape signals. A resistor is a component designed to provide a specific resistance.
Resistance is not always bad. In electronics, controlled resistance is one of the main ways circuits are made useful.