1)What does this graphhopper exactly do ? (In layman explanation please)
2) I have a project in which there are multiple APs and routers can I use them with Graphhopper to determine shortest path in indoors using multiple routers with signal strength as costs(just an idea). ?
If yes/no then how ?
I am totally new to this sorry for your interruption but need your help. Thanks 
If all you need is shortest path, I would suggest using a plain graph tool like https://github.com/jgrapht/jgrapht . Graphhopper is for integration with geospatial mapping data sets (like reading in open street map data and generating instructions). It looks like your problem is just a graph problem so this will be a little less complex.