An alkali metal is one of the elements in the first column of the periodic table. The alkali metals are all very soft, and all of them can be sliced with a knife. All of them are relatively light; lithium, the first one, is only half the density of water. In the elemental form, all of them are very reactive and strong reducing agents. In fact, they react explosively with water and thus must be stored under mineral oil under laboratory conditions. Alkali metals get more reactive toward the bottom of the column. Lithium can be found in lithium batteries and some medicines. The other alkali metals are not common in household items.
Sodium and potassium compounds are needed in the human body, while lithium can be slightly toxic.