Scrubbing and Seeking
Once you have placed Audacity in scrub play mode it will remain in that mode until you Stop 
and return to normal play mode.
The term comes from the early days of the recording industry and refers to the process of physically turning tape reels to move the tape past the play-head to locate a specific point in the audio track.
Scrubbing
Scrubbing works only in Select tool 
mode.
Pressing CTRL + left-click will put you into scrub play mode. When in this mode dragging the cursor right and left across the waveform will cause Audacity to scrub play forward or backward. Scrub play will temporarily stop when the green playback cursor 
(the "play-head") catches up with the mouse cursor but will restart again as soon as you move the mouse left or right.
The speed of the scrub play can be controlled by rotating the mouse wheel (if your mouse is so equipped). Each four steps of the rotated mouse wheel doubles, or halves, the speed - which is equivalent to one octave. The scrub speed will be shown, in yellow-ochre, superimposed on the waveform display when you change it with mouse wheel.
It is possible to zoom while you are scrubbing by holding the CTRL key down and rotating the mouse wheel.
Clicking the Stop button (or its shortcut Space) at any time will stop scrub play and return you to normal play mode. Alternately, pressing CTRL + left-click when in scrubbing mode will also return you to normal play mode. When scrubbing stops the scrubbing position can be marked with a left-click mouse gesture which moves the cursor to that position; you may then use CTRL + B to create a marker label at that point.
You can invoke scrub play while audio is playing normally and it will abandon the playback to perform the scrub play. You cannot invoke scrub play while you are recording or paused in recording mode. You can use the Pause button 
while scrubbing to pause and restart the scrub play.
Seeking
If you hold the Shift key down as you scrub the scrub speed setting is ignored. What is played back, though at normal speed, is many small slices of the audio and always something very close to the mouse cursor so there is no lag. This is similar to using the seek button on a CD player.
It is also possible to press and release Shift as you go thus alternating between scrubbing and seeking.
Smooth Scrolling Scrub
If you CTRL + double-left-click then drag to scrub, the green play cursor (effectively the "play-head") remains at the mid position of the waveform display and the speed of scrubbing can be varied much more smoothly by the position of the mouse cursor. In this mode it does not determine the target of the scrub rather it determines the speed of the scrub, the further away from the play cursor (in either direction) the faster the speed of the scrub. The scrub speed will be shown, in green, superimposed on the waveform display as you drag the cursor with the mouse.
Double-clicking also works with the Shift key held down making it much easier to skip through the track at a uniform rate as the amount that you skip depends on the distance between the mouse and the "play-head" green play cursor which remains still; you can easily move further from that line to accelerate, move back to decelerate or hold steady to skip at a constant rate.
