Imagine you live in Seattle and you have a blog about coffee. You're going to share a link with your readers to the website of a local coffee shop that serves the most amazing fresh coffee ever. Where would you place this link on the page?
If you really wanted your readers to see it, you'd position it somewhere obvious. Probably in the main body of the page, probably near the top of the page, and probably within some content that explains how amazing the coffee shop is.
You probably wouldn't place the link in the footer, right? Many users may not scroll down the page that far, and even if they do, they wouldn't expect to find useful links in that section.
Google is able to work out the position of a link on a page, and from this could choose to value it differently. If the link is in the footer of a page, then Google could reduce the value of that link because they assume it isn't a great link for users (otherwise, it wouldn't be hidden away in the footer).
Google can also use the position of links on a page in aggregate. For example, they could see if 50% of all the links pointing to your website are in some kind of footer. This could indicate low-quality link building, and Google may decide to take a closer look.
Another example could be if Google finds that 50% of the links pointing to your website are from sidebars. Again, on its own, this may be legitimate, but it could also be a signal to Google that you're buying links. Many link brokers will place links in the sidebar of pages as opposed to within the content.
Because of this ability, you should make sure that you are getting links from websites which are happy to link from within content wherever possible. There is nothing wrong with the odd sidebar link, but too many of them does not signal a good link profile.