ADDIE is a systematic instructional design model that consists of five phases - Analysis, Design, Development, Implementation, and Evaluation. The Analysis phase involves identifying the learning problem and goals as well as understanding the learners. In the Design phase, learning objectives are specified and instructional materials are designed. During Development, the instructional materials are created. In Implementation, the materials are delivered to learners. Finally, in Evaluation, the effectiveness of the materials is assessed through formative and summative evaluation to improve the materials.