Sometimes it is easier seeing a video of something in action than it is to read about and imagine how it works. Here are a couple of balance bike videos which show children actually riding the bikes.
Notice how young they are but are already very proficient at balancing on the bike while gliding down short grades. These bikes are great for learning to ride. The focus is placed on the balancing of the bike which is the most difficult part when leaning to ride.
They can be ridden in the grass just as easy as pavement. This also helps when learning to ride since some crashes are inevitable. The grass may provide a softer landing for the little ones the first few times on a balance bike. You can also check out this post to get some tips on how to ride a balance bike.