Search engine crawlers crawl the whole content on your webpage including images,links,keywords etc.Whenever they find a link,if the link is dofollow link then the crawlers follow the link and land on other webpage.
If you want to (i) get indexed in google and (ii) rank high on google then you have to focus on these dofollow
Nofollow backlinks are ignored by search engine crawlers.Whenever you are linking to any website which is not of high value then use rel=nofollow
attribute in your html of the webpage.
Hope quick addition to backlinks has been helpful.
Send me a PM
if you find it helpful or require further assistance.