In order to create a label in a LaTeX document, press C-c (
reftex-label). Just like LaTeX, RefTeX is context sensitive
and will figure out the environment it currently is in and adapt the
label to that environment. A label usually consists of a short prefix
indicating the type of the label and a unique mark. RefTeX has
three different modes to create this mark.
reftex-labelanyway is that RefTeX will know that a new label has been defined. It will then not be necessary to rescan the document in order to access this label later.
If you want to change the way certain labels are created, check out the
reftex-insert-label-flags (see Options - Creating Labels).
If you are using AUCTeX to write your LaTeX documents, you can set it up to delegate the creation of labels to RefTeX. See AUCTeX, for more information.
 Note that the context may contain constructs which are
invalid in labels. RefTeX will therefore strip the accent from
accented Latin-1 characters and remove everything else which is not
valid in labels. This mechanism is safe, but may not be satisfactory
for non-western languages. Check the following variables if you need to