In simple terms, the Software Development Lifecycle (SDLC) is a well-structured procedure to design, develop and test good quality software. The entire process is intricately developed step-by-step with the help of SDLC methodologies.
In software engineering models, SDLC helps in outlining each stage of development. This is essential as it assists the software development model in performing its task efficiently and delivering tasks on time following a cost-efficient approach.
For a detailed idea of what is Software development Lifecycle, read this blog till the end.