For years people have focused mainly on inbound links generally known as backlinks to help their site rank high on Google for their chosen keywords and prior to Google Penguin and Panda updates of late 2013 early 2014 this was how Organic SEO essentially worked.
Back in the day, before Google's infamous updates SEO used to be straightforward. A website needed very little on page optimisation because 90% of the work was done off page and involved creating 1000's of backlinks containing your selected anchor text , anchor text involved using your keyword as the back link text and the more Google found this anchor text linking to your website the higher you ranked... Head of Google's webspam team Matt Cutts however pointed on in this video that Google considered this to be spam...
Google was like the Wild West in these days and marketing companies simply created 100's of worthless and inane blogs filled with nonsense "spun" articles containing anchor text backlinks to their client's websites.
This was called black hat SEO marketing, lots of marketers did it knowing that it was wrong and Google was powerless - at the time - until they launched their Penguin and Panda updates which targeted black hat SEO, poor quality backlinks and predictable back link patterns i.e. 300 back links created in an afternoon.
After these new more complex algorithms where released, 1000's of websites dropped overnight and 100's of marketing companies found themselves on the wrong end of lawsuits after businesses lost revenue after these marketing companies had promised only to use white hat techniques. Google had guidelines in place and these back hat companies had ignored them so they where the one's at fault.
After 2014, some marketers claimed that Organic SEO was now dead and that Google had done this to promote its own PPC paid advertising programme known as Adwords and word went round blogs and social media that all back links where bad... Of course this isn't true, good back linking is still an important part of SEO, it needs to be done correctly and done naturally, but what many people, even people who claim to be so called marketing experts overlook is outbound links.
If you read Googles companies policy their goal is to give its users - the people using their search engine - the best possible experience and the correct information in relation to what they searched for, this is why SEO is more difficult nowadays, you can optimise a page title, description and meta data for a certain keyword but if a visitor instantly clicks back to the search results it tells Google that this page does not have the information the person doing the search wanted so puts your page further down the results for that search query.
This is why outbound links are important, they give your page more authority and show Google that your page contains an abundance of information about the topic or product that has been searched for.
Whether the information on your page or accessed through a link that you have it still helps with your Google ranking. Many webmaster are terrified of sending people away but they shouldn't be as you can open outbound links in new windows which keeps your page open and active in Google eyes.
Let's say you have a website about health and fitness with an article about the best exercises to lose belly fat quickly for the summer? Now amongst other things your article may tell people that water tablets can help with any bloating they might experience, so it would be a good idea to given people more information about water tablets, there side effects, the recommend dose etc without your article going off topic too much.
This is where out bound links work and are the type of outbound links Google loves.
In Summary, Don't be afraid to link to other sources and use citations where necessary and don't be afraid of quality link building