The phrase "in terms of" explains what standard you're measuring something on. In this example:

Whether they're growing in terms of revenue, I'm not so sure.

...the speaker wonders whether the competitor is "growing in terms of revenue". A company can grow in several ways, including revenue, profits, number of employees, number of stores, and in other ways. The phrase "in terms of revenue" explains how you're measuring the growth.

Other examples include:

A woman wants a guy who she can click with in terms of her interests & hobbies.

Where do you see yourself in 5 years in terms of your career?