Nofollow is an HTML attribute value used to instruct search engines that a hyperlink should not influence the link target’s ranking in the search engine’s index. It is intended to reduce the effectiveness of certain types of search engine spam, there by improving the quality of search engine results and preventing spamdexing from occurring.(i.e when using this attribute the hypertext (link embeded in words)on that page will not be followed by search engines to consider it as an backlink from that website.)
This attribute is written as rel=”nofollow” . When we add this attribute the link would not be followed by engines as an backlink and will not affect any rankings of that link.
And in reverse when we remove this attribute or didn’t use this attribute then all the links will considered as backlinks from the search engines and will affect the rankings of that link in search engine results .