The TEI header allows later users of the etexts you create to find out what the text is, who created the etext (i.e. you), and what source edition(s) you transcribed the etext from. In its full expansion, it also allows a full accounting of your transcription practice (did you correct typos silently? did you expand abbreviations? normalize spelling? etc.) and can also include a detailed characterization of the text itself (demographics of its author and audience, subject matter, genre, etc.) and a full change log, which is important for document management in large projects.
For bare-bones work, however, it's simplest to copy the following TEI header by rote, and replace the text in square brackets with appropriate information about the text being encoded. If the etext is not transcribed from a pre-existing source, but instead is being created in electronic form, the bibl tags within the sourceDesc element should be changed to p.
<teiHeader> <fileDesc><titleStmt><title> [Put the title of the electronic text here.] </title><publicationStmt><p> [Indicate who is publishing this electronic text (i.e. you).] </p></publicationStmt><sourceDesc><bibl> [Indicate the source from which this etext is transcribed.] </bibl></sourceDesc></fileDesc></teiHeader>
For example, the TEI header of the document you are reading looks like this:
<teiHeader> <fileDesc><titleStmt><title> Bare Bones TEI: A Very Very Small Subset of the TEI Encoding Scheme </title><publicationStmt><p> Published electronically by the Text Encoding Initiative, Chicago and Oxford, in 1994. </p></publicationStmt><sourceDesc><p> This text was created in electronic form. </p></sourceDesc></fileDesc> </teiHeader>