Tried flashing TWRP stuck at “recovery is not SEandroid enforcing”

Now when I try installing the custom recovery it just stays at the screen. I can boot normally & into download mode. But it won’t boot into recovery mode. I downloaded the latest version for my phone model and it still doesn’t seem to work. Any ideas?

Also, what is SEandroid?

And another question off topic. I usually use textdroider to change my dpi on my previous phones but it doesn’t seem to work for my s6. Any ideas?