A geometric series is a series where the ratio between successive terms is constant. For example, the series
$$
\sum_{i=0}^\infty\left(\frac{1}{3}\right)^i=1+\frac13+\frac19+\frac{1}{27}+\ldots
$$
is geometric with ratio $r=\frac13$.
The geometric series $\displaystyle\sum_{i=0}^\infty r^i=1+r+r^2+r^3 + \ldots$ converges to $\displaystyle\frac{1}{1-r}$ if $\lvert r\rvert <1$ and diverges if $\lvert r\rvert\ge1$.