The term "nonsexist language" refers to words and ways of speaking that do not imply that men are better than women. Because society (and especially the business world) has typically been male dominated, many of our speech mannerisms and much of our language does imply that men are better than women.
One of the examples of this that is often given is our tendency to use the pronoun "he" by default. When we write sentences about unidentified people, we tend to call them "he." This implies that anyone you are writing about in a business context must be male because men are supposed to work and women are supposed to stay home. The same is true of our use of the term "businessman." These are usages that are not meant to be offensive, but which clearly imply that business is a man's world, not a woman's world.
Nonsexist language, then, avoids these sorts of word choices. Nonsexist language might speak of "business executives" instead of "businessmen." It might alternate referring to generic people as "he" and "she." It will avoid word choices that imply that men are better than women or that it is more natural for men to be in the business world than for women to be.