What is a walkaway split?
You do that as opposed to what?
A split is a general term that means you turned one hive into two. There are lots of methods of doing this and one is a "walkaway" split.
The significance of a "walkaway" split is that you put some brood with eggs and some capped brood and some open brood and pollen and honey in a box and walk away. The bees raise a new queen and start a new hive.
Many splits are done with a purchased queen introduced to the split. This requires buying the queen and several times of checking for acceptance etc.
Somtimes they are done as "cut down" splits where you try to split into two hives with two focuses to keep them from swarming and to maximize production. The idea is to free the "production" hive from rearing brood and crowd them into the supers. The new hive is focused on brood rearing. Since one hive is short on foragers it won't swarm. The other hive has a fairly empty brood nest so it won't swarm.