Web design and development is basically an umbrella phrase that encompasses the creative process of developing a website.
Like the title says, it comprises two key skill sets: web design and development.
However, these titles often overlap, which means their meanings also vary from firm to firm.
In general, web design refers to the visual aspects of a website such as its layout, colors, images, etc.
It also includes elements such as user interface (interface that enables interaction with software and the rest of the site) and search engine optimization (enhancing the usability of a website by making it easier for users to find what they are looking for).
On the other hand, development deals with the functionality of a web page such as adding new features, bug-fixing, adding new functionality, etc.