Output is the net name of the output node in your simulation. Source is the signal source that you wish to use as the input from which to calculate the DC transfer function. To understand how to run successful simulations in EasyEDA please have a read through of the EasyEDA Simulation eBook and a play with the examples in there: https://docs.google.com/document/u/1/d/1OWZVVFRAe_2NW3WratpkA_SGuHa5AcRow5ZRfvcoVTU/pub (The Google link above is to the original copy of the Simulation eBook which you can also find at: https://easyeda.com/Doc/Simulation-eBook/ but the table of contents in the EasyEDA copy is broken and misses out some sections. We are working to fix it but in the meanwhile the copy published to the web from Google Drive works just as well.)