One Block. One Chunk. You start on a single magic block, confined to a single chunk. Everything beyond it is a forbidden zone - you literally cannot leave. Mine the magic block, build, and raise your island level: levels are chunk currency.
Mine the magic block, get more blocks
You can only build out the chunks you have unlocked. Unlocking required levels.
You need to sneek and hit the border again. This is a little hard to do if you are flying like I was in this screenshot!