Because pearls are natural organic substances, no two of them are identical. Therefore when a piece of pearl jewellery contains more than one pearl, some effort must be taken to insure that the pearls all match in an aesthetically pleasing fashion.
"Matching" here does not mean that all of the pearls are absolutely identical -- that would be impossible! Nor does it even mean that all of the pearls in the piece are generally uniform in size, shape, and colour. It basically means that the pearls "fit together" in a pleasing manner and that variations among them are either minimal, gradual or for a specific purpose. For example, a long strand of pearls may have a large pearl in the centre, with pearls which grow gradually smaller strung along either side. Or a strand may consist of pearls in alternating or gradually changing colours. In any of these examples, the pearls are considered well-matched if the resulting piece is uniform and consistent, rather than haphazard-looking.
In the same way pearl earrings, bracelets, pendants, pearl rings, etc. -- any piece which contains more than a single pearl -- are generally matched to achieve greater beauty and value. If the piece consists of contrasting colours (such as a two-pearl, black-and-white ring), the matching will take into account the size, shape and lustre of the respective pearls, even though their colours are quite different.
Pearl matching is really an art in itself. It requires a good eye, good judgement and a great deal of time and effort to match pearls effectively. Indeed, a strand made completely from natural pearls (rather than from cultured pearls) can require years to collect a well-matched set of pearls! Clearly good matching greatly adds to the quality and value of any piece of multi-pearl jewellery.