A and B in the following reactions are :
$R-CO-R' \xrightarrow{HCN/KCN} A \xrightarrow{B} R-C(OH)(R')-CH_2NH_2$

Step 1: HCN (with KCN as a source of $CN^-$) adds to the carbonyl group of the ketone to form a cyanohydrin (A):
$R-CO-R' + HCN \xrightarrow{KCN} RR'C(OH)CN$
Step 2: The product $R-C(OH)(R')-CH_2NH_2$ has a $-CH_2NH_2$ group in place of $-CN$. Reduction of a nitrile, $-C\equiv N \rightarrow -CH_2NH_2$, is done by $LiAlH_4$ (B):
$RR'C(OH)CN \xrightarrow{LiAlH_4} RR'C(OH)CH_2NH_2$
Hydrolysis with $H_3O^+$ would instead give the $\alpha$-hydroxy acid $RR'C(OH)COOH$.
So A = $RR'C(OH)CN$ and B = $LiAlH_4$.