Organic reactions are often shown in the following way:
Reactions are not usually balanced; the above should be considered a scheme rather than an equation. In fact, by-products are often omitted completely, especially if they are inorganic. A “reagent” is a chemical used to change an organic reactant into the desired product. There may be several steps, listing more than one reaction in one scheme, by showing each reagent/condition in sequence after (i), (ii), etc.
Here the alcohol is first treated with PBr3 and heated, then (in a separate reaction) it is treated with KCN in ethanol.
The value of this approach is seen when drawing out a multi-step synthesis. In such a scheme, it shows a starting material A being acted upon by a sequence of reagents (X, Y, then Z) in order to turn it eventually into D:
Notice that each arrow here represents a complete reaction, not just a step in a mechanism.