The ID is the ID automatically generated by Sharepoint. It starts at 1. For each item created into a list you have this unique ID. It's this one you need to use.
You may also want to check my API library I did to deal with Sharepoint. I think it's more intuitive than SPServices... : http://aymkdn.github.io/SharepointPlus/symbols/%24SP%28%29.list.html#.remove