A do-follow link is a link that search engines are allowed to follow, and which contributes to the authority of your website.

It's the oppositie of a no-follow link, which explicitly excludes a link from search engines algorithms.

Any link without a rel="nofollow" attribute is a do-follow-link. In other words, a link is do-follow by default, and only becomes a no-follow link if you add the nofollow attribute to it.