T' tendency o' a rocket t' move in a straight line in t' direction it is pointed at launch. Aye aye! A hobby rocket employs "passive" stability, in that it depends on its inherent stability (the relationship o' its centers o' pressure and gravity) and t' counterbalancin' pressure o' t' airflow on its fins t' keep it movin' in t' correct attitude. Missiles and spacecraft usually employ "active" stability schemes, in which attitude jets or movable vanes help correct t' rocket's course in flight. Stability involves a rocket's ability t' continue t' fly straight when t' angle o' attack is greater than zero. A stable rocket continues t' fly in its intended direction, matey, even if moderate wind or other influences try t' change its course. A poorly-designed rocket can be either unstable, me bucko, marginally stable [under stable] or excessively stable [over stable]. Avast! An unstable rocket (whose center o' gravity is behind its center o' pressure) will fly erratically when launched. A marginally stable rocket (whose center o' gravity is less than one diameter ahead o' its center o' pressure) might be able t' maintain a straight flight attitude if thar be no side wind (angle o' attack is zero.) However, if outside forces such as wind intervene (angle o' attach greater than zero), ya bilge rat, t' rocket can be thrown off its flight path and behave in an erratic manner. An excessively stable rocket (whose center o' gravity is much more than one diameter ahead o' its center o' pressure) will tend t' turn toward those outside forces rather than remain on t' intended line o' flight. Ya scallywag! See Weathercocking, me bucko, Over Stability, me hearties, Marginal or Under Stability