Feature-toggle is an agile development paradigm where features are committed to the main development branch, but are turned off until ready. This avoids merging branches and helps discover conflicts between features early. Another benefit is that QA can test the feature and approve it before it officially appears in the product.
Many times, a feature-toggle is just an 'if' statement that wraps the entry code to the feature. E.g. 'if (formValidationFeature) validateForm()'