A reactor is a piece of equipment where two or more substances react with each other to form one or more products. In the Haber process, nitrogen gas and hydrogen gas are fed into a reactor where the temperature and pressure can be altered to achieve the best yield. A reactor is usually thought of as a rigid, durable container so we can increase pressure and temperature without affecting volume.
A basic schematic of a reactor is shown below:
The two feed streams enter the reactor. A reaction takes place and one or more streams leave the column as the products, byproducts, or leftover reactant.