"Full sentences" means complete, grammatically correct statements that express a complete thought and usually follow a subject-verb-object structure. Full definition
One who can't be bothered with speaking in full sentences until bedtime, when he becomes the most vocal, social, charming little person in the land. (mommyish.com)
Each 5th grade reading passage is followed by 5 - 6 questions which the student should answer by writing full sentences in the space provided. (k5learning.com)
It's also important to use full sentences in letters and emails. (targetjobs.co.uk)