Resistors (Ω) oppose current, are color-coded, and connect in series (Rt = ΣR) or parallel. Current-limiting resistors protect LEDs.

As explained by Arduino to Go, each color band on the resistor corresponds to a number but this can differ depending on the band's placement. It's not complicated, but the sheer number of colors ...

When resistors are connected in parallel, the supply current is equal to the sum of the currents through each resistor. In other words the currents in the branches of a parallel circuit add up to ...