What is SNDBOX ?

I see a lot people posing around this SNDBOX!
As lIke this Images Source File .
snddd.png
https://steemit-production-imageproxy-upload.s3.amazonaws.com/DQmRyBfNodb8XMVL3NRtjdv9PWbCxbB9FuWSVNZTLjRB51Y

Is it a completion?

H2
H3
H4
3 columns
2 columns
1 column
Join the conversation now