Resistance has to do with where there are sticking points in a share prices movement. So, on the way up, a price would have a hard time pushing through resistance, but traditionally on the way down, that same price level will act as support to keep the price from falling. This is a result of reinforcing effects.

For example, say in the past there was a huge limit sell order at or around $0.42. more people will start to put orders around that price so that they can cash out there profits before resistance is hit hard, and the price starts a negative trend. Over time, these persistent sticking points become more prominent. It is significant when a price breaks through resistance because it is generally done on high volume, and is often met with limit buy orders once it is through that price level. This can propell a stock upward with much velocity causing significant price action.

All the best,

David