What Pagerank and Link Juice Is:
Each page on the Internet has a ranking in the eyes of the search engines, based on over 50 ever-changing factors, such as how many pages link to that page, how high the pages rank that link to it, and how much time people spend on the page after searching for it, before returning to the search engine.
Whatever the page's ranking is, if there are outgoing links on that page, they can either leak out that ranking, passing it to the page being linked to, or they can keep the ranking from leaking out. If you have a link on a high ranking page, you want that link to share its link ranking with you, but if you have links to other sites on your high ranking page, you probably want to keep that ranking from leaking out. This ranking is commonly called "pagerank" and the sharing of pagerank is commonly called sharing "link juice".
How Redirect Links Affect Page Rank:
There are a few ways to control link juice. One way is by setting the link to be a 301 redirect or a 302 redirect. These are just web server codes attached to your redirect links that tell search engines how to treat your link.
A 301 redirect tells the search engine that the redirect link is permanent and that it doesn't need to check to see if the link has changed very often. It also tells the search engine to send link juice from the sites that link is published on to the destination site you specify in Tracker.ly. This benefits your destination sites at the cost of the sites you're publishing your links on, so you want to set your tracking links to 301 if you're publishing them on other people's sites and directing them to your sites.
A 302 redirect tells the search engine that the redirect is temporary and that it should check back often to see if the link has changed. It also tells the search engine to keep the link juice on the page your links are published on. This benefits the page your link is published on, so you want to set your tracking links to 302 if you're publishing them on your own sites and they direct traffic to sites that are not yours.
Since you can create many tracking links to each destination, you can tweak them depending on where you are using them to help maintain or grow your pagerank. The higher your pagerank, the higher it will appear in search engine results.
How to Change 301/302 Settings:
You can set the overall default for all NEW redirect links by clicking the "Default Link Juice" button on the first page of the wizard, under "General Settings". Clicking it will toggle between 301 or 302, and the button will tell you if it is benefiting the referrer's page (where you're publishing the link) or the destination page (where the link goes to).
You can override the overall default setting and set the default for each destination by going to the management page for the destination and clicking the "301/302" button there. It says on the button what the default is currently set to, and clicking it will toggle it, just like the overall default above. So, if this destination is on one of your sites, you probably want to change it's default to 301 so that all the redirect links are sending link juice to your site.
Finally, you can set it on individual tracking links by clicking on the link juice button on the Manage Redirect page in the wizard for that redirect link. So, if you're publishing the link on your site, you probably want it to be a 302, so that you don't leak any link juice off your site.