What's going on here? I'm trying to apply a normal edge split with a subdivision to make it look high poly with good edges. But when I applied it on this model it did this.
Edge splitting doubles the vertices and edges, effectively creating to separate parts, which then get subsurfed individually. If you need sharp edges, adjusting the Mean Crease is likely a better way. It's under Transform in the Properties panel while in Edit mode.
Here I have all the edges of the Icosphere selected, so they all get the same Crease value. You can, of course, adjust the crease for each edge individually. It applies to the currently selected edge(s).
The Edge Split modifier makes two duplicates of each edge that is greater than the Split Angle: value, one for each face that touches the edge. This makes it so that when you add a subsurf modifier, set to Catmull-Clark, it treats most faces as their own plane, since the faces are not actually connected anymore. Like, have you ever tried adding subsurf to a plane and seen it turn into more of an octagon? It's like that.
What I can recommend as a solution for you is to add just a subsurf modifier and then use a lot of loop cuts (shortcut Ctrl + R) in Edit Mode to tighten the edges. If you use loop cuts, you will still get nice clean edges, just like you would with the Edge Split modifier. Read more about loop cuts in the manual.