"Attach to Process" command (Ctrl + Alt + P) has been moved to Debug
menu in Visual Studio 15, in earlier version it used to be in Tools menu.What is Attach to Process?
Generally developer uses "Attach to Process" command is used to attach the processor (.exe) file in visual studio that let them debug the code without running the project from Visual Studio.
First run the application by right clicking and View in Browser. Then go to "Attach to Process" dialog and select "iisexpress.exe" file and click "Attach" button. Now go to respective page in the browser where you have kept break point and you will notice that debugging starts. Sometimes you may notice that debug doesn't work in this way, then you can rebuild the application and follow the same steps.
In case your have hosted your application on local IIS and you want to debug then instead of "iisexpress.exe" attach "w3wp.exe" file.