It means that you can’t view NSFW blogs like this one unless you’ve logged in and chosen to turn off ‘safe mode’
https://staff.tumblr.com/post/162047130530/safe-mode
The idea is to lock down some of the more salacious content Tumblr has so you have to have actively chosen to view it. I guess this ties in with its sale to Verizon, so potentially some rocky seas ahead for us kinky blogs, who can tell.
It also stops those who have entered their age as under 18 from accessing any ‘unsafe’ content. I’m sure you see the same flaw in that system as everyone else does.
Personally I haven’t seen a drop off in figures for those viewing the blog since it started in July. I thought my ‘bounce’ rate might go up (the number of people who visit the site and then leave straight away from Google).