as_label() transforms R objects into a short, human-readable
description. You can use labels to:
- Display an object in a concise way, for example to labellise axes
in a graphical plot. 
- Give default names to columns in a data frame. In this case,
labelling is the first step before name repair. 
See also as_name() for transforming symbols back to a
string. Unlike as_label(), as_name() is a well defined
operation that guarantees the roundtrip symbol -> string ->
symbol.
In general, if you don't know for sure what kind of object you're
dealing with (a call, a symbol, an unquoted constant), use
as_label() and make no assumption about the resulting string. If
you know you have a symbol and need the name of the object it
refers to, use as_name(). For instance, use as_label() with
objects captured with enquo() and as_name() with symbols
captured with ensym().