Hobby computers are computers that involve some sort of pastime that is not professional, but which gives the user pleasure - just like any hobby.
This could mean software to create art, designs or games themselves.
It can also refer to wargaming - playing some fantasy computer war with other users on the internet.
It could also refer to some virtual world. Whole worlds have been created on line with people assuming roles in a fantasy world.
Some of these hobbies do not involve software development - only a knowledge of the application software. Others, such as artwork and design, require an extensive knowledge of software development applications. These are what are referred to here, I believe.