Beach - for simplicity (Ufer) - it is a piece of rebar bent upwards (i.e. coming out of the poured concrete footing/foundation) - this piece of rebar is usually placed pretty close to your main panel so it is easy to bond the panel to (using a #4 copper conductor)...see 250-66(b)- "Concrete Encased Electrodes" is referring to a Ufer (as above named after the inventor of this system)....do check out the link above as it gives some good history behind this system.
