Alpha masks on sprites?

Hey guys,

I'm relatively new here, but I have a question about how I might go about doing alpha masking with 2D sprites.

From Photoshop I'm used to creating this effect via a layer mask, if that helps describe what I'm doing. Here's an example:

https://imgur.com/9k8l77l

I want part of the tree to be partially transparent while the rest of the tree is completely opaque.

From a little bit of research, I'm guessing I can do this with shaders, but I don't know much about them.

I found this example of someone doing what I'm trying to do, but I don't have enough knowledge to unpack what's going on or how to use it.

http://answers.unity3d.com/questions/766325/dynamic-alpha-gradient-on-sprite.html

Any advice or help is greatly appreciated. For what it's worth, I'm basically trying to craft a technique so that, when my player is behind an object, part of that object (where the player is) becomes partially transparent.

Thanks in advance!