Tollen's reagent is a solution of ammoniacal silver nitrate and generally used for the detection of $-\mathrm{C}\mathrm{H}\mathrm{O}$ group. Aldehydes reduce Tollen's reagent and itself get oxidised to give $\mathrm{A}{\mathrm{g}}^{+}$ ions to Ag powder which forms the silver coloured mirror in the test tube. So this test is also known as silver mirror test.
$\underset{\left(\mathrm{P}\mathrm{o}\mathrm{w}\mathrm{d}\mathrm{e}\mathrm{r}\right)}{\mathrm{R}-\mathrm{C}\mathrm{H}\mathrm{O}}+\left[\mathrm{A}\mathrm{g}\right(\mathrm{N}{\mathrm{H}}_3{)}_2{]}^{+}\to \mathrm{R}-\mathrm{C}\mathrm{O}{\mathrm{O}}^{-}+\mathrm{A}\mathrm{g}$