One type of hyssop is a small, hardy shrub. Another is an herb from the basil family, somewhat bitter and mint-flavored. Tourists to Israel are purportedly sold this "hyssop" in small jars, though since it is a protected plant in Israel the jars frequently contain a blend of other Mediterranean herbs. It has also been suggested that hyssop is actually the caper plant, a green, rock-climbing vine that is said to have cleansing properties.
In any case, the hyssop plant in the bible symbolized spiritual purification (cf. Ps. 51:7; Heb. 9:19).
-Today in the Word, p.21