Should I disable user zoom?

Usually no. Disabling zoom hurts accessibility, so keep user-scalable enabled unless you have a strong reason.

What does width=device-width do?

It matches the layout viewport to the device screen width, the standard for responsive design.

Generate the responsive viewport meta tag.

width=device-widthInitial scaleMinimum scaleMaximum scaleAllow user zoom

Output

<meta name="viewport" content="width=device-width, initial-scale=1, minimum-scale=1, maximum-scale=5, user-scalable=yes" />

Configure scale and zoom options to produce the <meta name="viewport"> tag for responsive pages.

Should I disable user zoom?: Usually no. Disabling zoom hurts accessibility, so keep user-scalable enabled unless you have a strong reason.
What does width=device-width do?: It matches the layout viewport to the device screen width, the standard for responsive design.