Link juice is the link building credit that you get when you link your webpages with other reputed pages.
E.g. You have two backlinks coming from PageA and PageB. PageA have 20 backlinks from reputed websites and PageB have 11 backlinks from less reputed websites.
In this case you will get more link juice from PageA.
A link is assumed to pass more link juice if:
it does not have the nofollow attribute added
the page the link is on is indexed by the search engines
it is not detected by the search engines as a paid link to manipulate the rankings
the link is not part of a link exchange
the number of links on the page that the link is on are low
the page that the link is on, has a lot of link juice pointing at it
the link was editorially given and not a self promotional easily gained link
the page that the link is on is relevant and on the same topic as the page that the link is pointing at
the anchor text of the link is the same topic as the page that the link is pointing at
the site that the link is on is recognized as an authority site by the search engines
the link is in the context of some text that the pages the link is pointing at is about
a page with a higher mozRank and mozTrust is likly to pass more link juice