A multistage rocket is one that uses two or more engines that ignite consecutively (one after another) to propel a rocket higher and faster. Lower and intermediate stages always use engines that have no delay or tracking smoke and no recovery system ejection charge. The absence of a delay charge allows the next engine to receive the maximum velocity from its propellant burn. These engines are referred to as booster engines and they are coded with a zero where the delay charge is noted (i.e. C6-0).
In the upper final stage, an engine with a delay and tracking charge and recovery system ejection charge is used. An engine with a long delay should be used as the rocket must lose velocity before activating the recovery system. This will give you greater altitude and avoid damage to the recovery system.
Before attempting to build a multistage rocket, you should build and fly several single stage rockets to familiarize yourself with the principles involved. The reliability of a two stage rocket is always less than a single stage rocket; as more stages are added the reliability becomes less. Hence, more building and flying skill is required as model rockets become more complex.