Group 1 of the Periodic Table, or the Alkali Metals, include lithium, sodium, potassium, rubidium, caesium, and francium. The columns of the Table reflect a "family" of elements; each member within the family has the same number of electrons in its outer shell, but those electrons are further from the center of the atom (nucleus) going down the column, as a general rule. All atoms tend towards a completely full outer shell, some achieving this by shedding electrons, others by gaining them. In the case of the Alkali family, each possesses a single electron in its outermost shell, which they want to "give away" to achieve that more stable outer shell, which makes them very reactive with other atoms or compounds that want an additional electron in their outer shell to achieve stability.
The metals in group 1 are known as the alkali metal. These metals are so reactive because they all have 1 valence electron and they want to "get rid" of this one electron in order to gain a pseudo noble gas configuration, therefore making them really reactive with other atoms "looking" for an electron.