I looked it up on github, this is the line in the code:
https://github.com/Arcitectus/Sanderling/blob/master/src/Sanderling/Sanderling.Exe/sample/script/Mine.ore.cs#L110
Here is what I read from the C# code:
This line declares a property called ShieldHpPercent with only a getter function. This C# specific terminology aside, you can view it as just a function. This means, when ShieldHpPercent is used in an expression somewhere else, this results in a function call.
An example of this can be seen on the following lines:
https://github.com/Arcitectus/Sanderling/blob/master/src/Sanderling/Sanderling.Exe/sample/script/Mine.ore.cs#L118-L120
As you can see in the example above, the function call for a property is coded without parenthesis, so its less obvious that it is a function call.
For more background on this, I recommend the following documentation: