Short scale basses are just that... shorter in scale (the distance from nut to bridge). In most cases around 30". Full scale basses are around 34". By comparison most LP style guitars are 24.75" scale and Strat and Tele types 25.5".

Short scale can be a good transition from guitar for a beginner, or more comfortable for youths or someone of smaller stature, or just more convenient for travel etc. I started on a short scale bass at 12 yrs old and "graduated" to full scale in my late teens.

As with many things guitar and gear related, you'll have to try a few out to see what you like.