Monitoring
"Oh, how I love to hear myself louder!" - Some overjoyed musician. 

Some sound devices allow you to hear what you are recording.  Most USB microphones do not automatically allow you to hear what you are recording, though.  To hear or 'monitor' a track, click the arm recording selector button followed by clicking Monitor Incoming Audio

We recommend that you use headphones in order to avoid feedback.

One caveat to this is that you must set your sound card to mix with a low latency, otherwise it will sound like an echo chamber and the sound could be delayed by seconds.  You can go into the preferences and try ASIO mode, if available, or reduce the buffer size to 4096 or 2048 and reduce the number of buffers to 2 or 3 in Wave mode.  The down side to reducing your buffers and buffer size is that you may hear gaps in the audio, depending on your sound card and computer speed.

Hearing Effects In Real Time
If you add effects, you will also hear the effects on a track if monitoring is on.  The effects are not recorded and you can change them or remove them later, without affecting the recording.