Using “Union”

I am a retired, part time hobbyist with limited knowledge. I really have enjoyed making stuff with Shapr3D, but sometimes I get stumped.
When drawing my shape, there are a lot of individual parts the can be extruded separately. Do I have to use the “union” function to get them all to stick together so it prints well? I have had a couple errors -
“Target or tool is a general body”
“Non-manifold body”
I am not sure what they mean or what to do. Or even if it really matters ?

Hi- glad that you like the app. These error messages mean you have created non-manifold bodies: If this happens the best thing to do is start over again, as you unfortunately can’t undo this with any boolean operations.

Can you post a screenshot?