Link juice is basically the influence created by the backlink. You can say link juice as the gold of Google. In other words you can say that it is the secret to beat your competitors in Google Rankings.
Link juice is the most important factor to dominate Google. You have to understand Google to know what link juice is.
This rule also applies in internal links. If you linked 10 different pages on your single page then link juice will be divided. If you want maximum juice for any page so you must backlink single page.
The link juice is also determined by different factors, where is link is located, the PageRank, how many links are on the page, how much the link is relevant etc.
Dominated sites in Google Search, have millions of links from other sites, who have hundreds of thousands of links from other sites who have links from other sites who have links from other sites.
If you want to rank #1 for your keyword, you should get more link juice than any other sites that currently show up for that keyword.
To increase link juice you need to get more backlinks, especially from sites that have a lot of juices.