Change how wide a stereo file sits, move it left or right, and keep the low end centred. All three work on the mid/side pair rather than on left and right directly.
Mid and side
Any stereo pair can be written as a mid channel, which is what the two sides share, and a side channel, which is what differs between them. Mid is (L + R) / 2, side is (L - R) / 2, and adding or subtracting them returns the original pair.
Scaling the side channel is what changes width. Nothing is added to the file, so a recording with two nearly identical channels has almost no side content, and widening it does very little.
Width
At 100% the file is unchanged. Below that the image narrows until 0%, where left and right are identical and the file is mono in content, though still two channels.
Above 100% the difference between the channels is amplified. It is worth a check in mono: adding side content that cancels when the channels sum leaves a phone speaker with less than it started with.
Pan
Pan uses constant-power gains, so the centre position is unity on both sides and moving off-centre keeps the perceived level steady rather than dipping in the middle. Full left mutes the right channel.
Pan runs after width, so a narrowed file can be placed anywhere across the field without the two controls fighting.
Bass to mono
The bass-mono control removes the low band from the side channel, below the frequency you set, which centres the low end while leaving everything above it as wide as it was.
Two practical reasons to use it. Low frequencies that differ between channels partially cancel on any system that sums to mono, which includes most single-speaker devices. On vinyl, out-of-phase bass drives the cutting stylus vertically and risks the needle jumping the groove.
100 to 150 Hz is the usual range. At 0 the low end is left exactly as it was recorded.
Example: a slightly narrow acoustic mix
A live stereo recording with the pair set close together sounds centred and small.
Set width to 130% and bass-mono to 120 Hz. The room and the reflections open out, while the low end stays anchored in the middle. Drop the width back to 0% for a second to confirm the mono sum still carries the whole performance.