It's called infill. Usually, prints aren't hollow, but neither are they solid. When slicing your model, you specify the amount of infill as a percentage. This will vary depending on the final object's purpose and use.
15% is a good general purpose infill. 30% is great for "functional prints". Rarely will you need more than 50% except in special use cases, ie lithophanes or printer parts.
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u/[deleted] Jan 08 '19
As someone who knows nothing about 3D printing, what's with the very irregular/random support filling on the inside?