You move left and right, trying to avoid zombies and trees. That's it. Why half a star though?
Well first off, the character's running animation is just two frames. But even those two frames are pretty bad, because the lighting of the character changes in each frame. This really made the character look bad.
And then you got the trees. You can see that they're moving down and that the ground isn't. This is pretty off-putting if you ask me.
On top of that, you got the music. The track is really small and it gets cut off when it's trying to loop. This is really annoying honestly.
Lastly, I would like to mention a nitpick of mine: the ammo section on the bottom. This isn't really a flaw or anything, I just wanted to mention that it really sucks that it is there, because (as of now) it doesn't seem like I can shoot in this game at all.
Judging from your author comments, I can see that you're open to criticism. So I hope my criticism helps you to improve the quality of your future games!