An X resource is a key value pair that is stored with the X server. For the X server, the value is meaningless - it's up to the clients to parse and understand it.
Usually, such values are used to tune the appearance of client applications: colors, fonts, cursors, themes etc.
Format
The format for an X resource is.
application.component.subcomponent.subcomponent.attribute: value
or
name.class.resource: value
The first part (application … attribute) is the key.
xrdb
Resources are managesd with the xrdb program.
A resource file (for example ~/.Xresources) is loaded with xrdb resource-file (usually xrdb ~/.Xresources). This gets rid of the old resources and replaces them with the new set.
In order not to get rid of old resources, xrdb -merge resource-file should be used.
The X resources currently being in effect can be queried with xrdb -query.