Literally, a bikeshed is a small outdoor shelter into which one may store one's two-wheeled form of transportation. However, in FreeBSD parlance, the word is a derogatory term that refers to any oft-recurring discussion about a particular subject; in particular, it is most often used to refer to a topic which has never reached a consensus within the FreeBSD community, and instead remains controversial. (The genesis of this term is explained in more detail in this document). You simply must have a working knowledge of this concept before posting to any FreeBSD mailing list.
More generally, a bikeshed is a topic that will tend to generate immediate meta-discussions and flames if you have not read up on their past history.
Please help us to keep the mailing lists as useful for as many people as possible by avoiding bikesheds whenever you can. Thanks.