Blend At Marker in the menu.

Use Blend At Marker to smooth the transition between edit points. After cutting or pasting, the waveform at the selection endpoints may not match, causing a pop or click. Blend At Marker takes a small section of audio on either side of the marker and overlaps it with fading so that the section on the left is faded out and the section on the right is faded in. This ensure a gradual transition between edit points.

Note that the length of the file is reduced because of the overlap. When blending music, audio tracks from videos, or other time/tempo sensitive audio, be sure to compensate for the blend duration.

To crossfade between two songs with more control over the fade shape and duration, use instead.

Blend At Marker Settings
Setting Description
Duration of blend (s) Sets the amount of time to overlap on each side of the edit marker. A value of 1.0 takes one second on the left and one second on the right and blends them, creating a one second blend. The length of the file decreases by one second due to the overlapping of the two parts.
Marker Sets the markers affected by the blend. After pasting in the middle of a file, both the Start and Finish markers may need to be blended. After cutting audio, only one the Start marker needs to be selected.
Curve Sets the fade curve for blending. For music and highly varying audio, choose "Equal power". For uniform audio, such as pure tones, choose "Linear".