Accuracy refers to how correct learners' use of the language system is, including their use of grammar, pronunciation and vocabulary. Accuracy is often compared to fluency when we talk about a learner's level of speaking or writing.
A learner might be fluent (make their meaning clear) but not accurate (make a lot of mistakes).
In the classroom
Language manipulation activities can help develop accuracy. These include controlled practice, drills, the study and application of grammar rules, and activities that help students to 'notice' their own mistakes.