It should return all the visitors where 'complete' event was set to 1.
Order equals 1 checks for visitors where purchase was done once.
Note: Events can be set to 'X' integer. So if you are create such segments, it will looks for hits/visits/visitors where the specified condition was met.
Orders=1: means that this segment will filter out and only show those visitors who have ordered only 1 item or added a single item to the cart.
Complete=1: means that this segment will show those visitors who have completed 1 order or bought a single product.
I hope this answers your query.