The default line width is dependent on the device used for the plotting. In order to read this information, you should be aware of the general interpretation of inches, points and pixels. The general rules are:
a point is 1/72 of an inch
a pixel is standard 1/96 of an inch, or 0.75 points.
This can depend however on the settings of your device:
the pdf() and postscript() devices:
standard a point is seen as 1/72 inch
lwd = 1 refers to a linewidth of 1/96 inch or 0.75 points.
the png(), jpeg(), tiff() and bmp() devices:
standard a point is seen as 1/72 inch
this can be chanced by setting the argument res which defines the ppi (points per inch).
lwd = 1 is 1/96 of an inch but takes the settings of res into account. So if res = 96, lwd = 1 is a thickness of 1pt, but your point is interpreted smaller than a regular point.
Also keep in mind that with the bitmap devices, a higher setting of res will blow up your figure on screen. So in the previous example, a lwd = 1 with a setting of res = 96 gives a line with a thickness of 1pt, but shown on the same screen this will increase the line thickness from 1 pixel to 1.33 pixels. If you scale both figures to be exactly the same size, they look the same (apart from the resolution obviously).
See also:
How do I specify axis thickness in a plot? (in R)
Increasing the thickness of box lines in an R boxplot?
https://graphicdesign.stackexchange.com/questions/199/point-vs-pixel-what-is-the-difference