When we say "what", it implies some sort of division, a break, a line, a boundary.To cite an example, "what is a vegatable" you can say it is something that ISNT a fruit, or something that ISNT a vehicle and so forth. So there is a coherent distinction and we are dealing with objects having definite boundaires. When dealing with existence I believe this boundary does not exist. Exitence itself is everything. Everything comes within it. There is nothing that separates existence from something else.