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.
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!
Mine the magic block, get more blocks
You can only build out the chunks you have unlocked. Unlocking required levels.