An ascending channel is a trend channel that is trending higher, aka Uptrend Channel. The channel highlights the stock's price action by drawing the lower line on pivot lows (troughs/channel support), and the upper line is the channel line drawn on pivot highs (peaks/channel resistance).
Prices don't always stay inside the channel. There can be quick pops above channel resistance and/or quick drops below channel support. As long as the action does not stay outside the channel for too long with respect to the time frame you are looking at, it holds as a valid pattern. For example; if you're looking at a daily chart, the breaks should not stay outside the channel for more than 2 days.
Furthermore, a break above channel resistance with strong buy side volume can indicate an acceleration of the uptrend. Also, a break below channel support with volume can indicate a reversal of the Uptrend Channel that is in tact.
Here's a visual example of an uptrend channel via a 60 day chart of AAPL. In this example, the stock manages to stay within the channel the entire time. This is fairly common.
Related terms used in stock trading and options trading: Descending Channel, Downtick, Uptick, Downtrend, Uptrend