TL;DR - Unless you know what you are doing, leave echo cancellation on. However, the best audio quality can be achieved by disabling echo cancellation and preventing audio leakage at it's source.
Zencastr has a built-in echo cancellation that helps quiet echos caused by feedback between the headphones and your microphone. It works pretty well in most cases but if you end up talking at the same time as your guest, it can cause the audio to sound muffled briefly in the recording. Echo cancellation helps cover up the problem but doesn't prevent it at it's source.
If you notice that you are having issues with this sort of distortion, you can disable echo cancellation in the audio settings. Doing so will prevent the muffled distortion but you might then hear and record the echos. If you do this, you must be extra vigilant about combating audio leakage that causes the echos. If you can hear the echos, they are also being recorded.
In general, you can fight audio leakage and echos by making sure that everyone is wearing headphones and that their volume isn't so loud that audio is leaking out to the microphone. Here is a blog post with more details on this: http://blog.zencastr.com/blog/2015/8/5/tips-for-avoiding-audio-leakage-when-recording-your-podcast