Channels can be filtered to look for keywords or show only posts which contain photos or videos (right now only instagram videos are supported). Filtering is useful for finding posts with specific content especially posts with only photos or videos.
One thing to note is that as you load more posts in the channel, if the filter is enabled it will continue filtering.
At the top of channels you may see a notice that says "Entries Hidden" which means that the channel is hiding posts with banned content and retweets. If you click the "Entries Hidden" button it will reveal the hidden posts in the channel.