In other words if you use links in your post so that Google will index them and allow others to find what you link when using Google, until you reach TL3, Discourse it telling Google with rel="nofollow" that your links are not of importance, so do not index them.
Admins have a stats page available to them to see your progress to a trust level, e.g.
To reach certain trust levels you have to have a minimum number of likes given.
Also the stats are calculated within a moving 100 day window. If you don’t keep your stats up you can lose TL3.
Also be aware that while you can see counts of stats such as Likes Given in your public profile, the counts in your public profile may be greater than those used for TL3 calculation because the TL3 calculates the requirements using the last 100 days.
In other words, a like created more than 100 days ago does not count toward earning/keeping TL3.
I find the nofollow on links to be strange. It’s mainly to prevent spam influencing page rank. But spam is unlikely in our group; it’s best to let Google/Bing/whatever figure out how to weight links rather than trying to apply crude adjustments.