Tertiary level of industry is the part of the economy that provides service to its consumers. Example: schools, restaurants, financial institutions, transports. It is divided into two segments: profit (they focus on generating income) and the non-profit (like government-owned schools).
The other two levels are the primary level which focuses on the harvesting of raw materials, which will be bought/given to the secondary level, the one that produces goods and up to the tertiary level -- the one that delivers these goods or services to the customers.
So it's basically an hierarchy/ladder-like/step-by-step thing.
Hope I was able to help. :)