A blog is a discussion or informational website where we can share different informations about a lot of important things. Things about our daily life, about business, about work, about society. But different from the rest of your site the blog section needs to be frequently updated with new posts.
Blog posts allow you to rank in search engines for a variety of keywords that you use in a post. For example if someone search on Google with one of the keywords of your blog topic then you will be part of the search result.
Furthermore your blog is a tool that allows you to be more involved with your audience, either by analyzing how many readers share your posts on social media or by allowing readers to comment on your posts. That way, a blog is more like a bridge between you and the audience in your website.