Iambic tetrameter is a meter in poetry. It refers to a line consisting of four iambic feet. The word "tetrameter" simply means that there are four feet in the line; iambic tetrameter is a line comprising four iambs. The Green Lantern oath (as well as the oaths for the other corps) is written in iambic tetrameter.
Some poetic forms rely upon iambic tetrameter: triolet, Onegin stanza, Memoriam stanza, long measure (or long meter) ballad stanza.
Example
Ap PAR..|..elled IN..|..cel EST..|..ial LIGHT,
The EARTH,..|..and EV..|..ry COM..|..mon SIGHT,