Lets start with the basics (from the Concordance of Arcane Space, AD&D Adventures in Space, Spelljammer Boxed Set):
Everything you know about space is wrong.
Infinite space; stars as flaming spheres of super-heated plasma; movement through space as a balance of scientific foces, thrust providing acceleration and maneuverability; scientific fact backing up natural phenomena; life on other planets built along blocks of carbon or silicon elements.
Forget all that. Its wrong.
You can get out of the atmosphere on the back of a roc; fly between the planets through a breathable ocean of air; sail between the crystal spheres that surround the inhabited worlds on a river of magical energy; encounter roving mind flayers and beholders. The stars are living things in some areas, great bowls of fire in others, and pinpoints of light painted inside a sphere in others.
One of the best part about spelljammer is is campy throw back to medieval style physics: the stars as points on a giant sphere in the sky, gravity? everything just naturally sticks "down", and the great rivers of phlogiston (what medieval scientists thought was burned in candles and other fires)...
Sure it requires some serious suspension of disbelief, but not any more than mages throwing fireballs nor the gods prancing about... plus much of it comes from things people really did believe hundreds of years ago.
If you've got any questions about how all this sailing through space stuff works, let me know.
You could also grab the Arcane Space Tileset from the vault and check it out for yourself