A field called "reference" appears in several places within the KiCad system, and carelessness with this can "break" you project.
This page has been checked to see that confusion in my brain (before 9 November, 2011) about the roles of pin names and pin and pad numbers hasn't led to errors here. I have a page with more on this if you are really curious.
The data in the field called "reference" for things inserted into the schematic (using eeSchema) is an element of how KiCad keeps track of the different devices used in your project. The things in your PCB artwork also have "reference" fields, and, unless you've messed things up, there will be a 1 to 1 correspondence. I.e. if your schematic has an "R1", your PCB will need to have an "R1"… but to make the system powerful, ways for you to derail things exist.
The data in the reference field must be one "word", ending in a digit.
R100 « You would not use this to suggest the "size" of the resistor.
("R100" would arise if you had more than 99 resistors in your project.)
R1a «doesn't end in a digit
Power On LED1 « not "a word". Note: We used underscores above.
When in in response to a right click, you get a "Clarify selection" response, one of your choices is "Field Reference". If you click that, you are electing to work with the field called "reference", not to work with some other "reference" to some "field".