It's been blogged and documented about hundreds of times before but, to quote https://engineering.purdue.edu/ECN/Support/KB/Docs/UsingTheXdefaultsFil/?pp=1

The exclamation mark (!) or /.../ is used to comment a line in the .Xdefaults file. For example:

 /* this is a comment */

Instance names are considered more specific than class names. If you use both the class and instance names of a program to specify a resource with different values, the instance resource value will be used. For example:

xterm*Background: blue

will be used, instead of:

XTerm*Background: green

To reread your .Xresources file, and throw away your old resources, you can type:

xrdb ~/.Xdefaults

To reread your .Xresources file, and keep your old resources, type:

xrdb -merge ~/.Xdefaults

This will merge any new resources you've put in with the currently active resource settings.

Avoid blank spaces at the beginning and end of lines in the .Xdefaults file.

You can change the instance of many X programs by starting them with the -name option. For example, if you start an xterm window by typing "xterm -name XT1", then you can specify resources for it in your .Xdefaults file with "XT1,", like this:

XT1*Background: yellow