This type of triangle chart pattern occurs when there is a resistance level and a slope of higher lows.
What happens during this time is that there is a certain level that the buyers cannot seem to exceed. However, they are gradually starting to push the price up as evident by the higher lows.
In the chart above, you can see that the buyers are starting to gain strength because they are making higher lows.
They keep putting pressure on that resistance level and as a result, a breakout is bound to happen.