Smoothing is the grinding down of the metal surface of the fields on a coin (usually bronze) to "smooth" pitted, rough areas.  Smoothing can also be performed on areas other than the fields, however, if the work modifies the original form of the reliefs or attempts to recreate worn or corroded details, then it must be described as tooling, not smoothing.  Smoothing is a form of tooling but is less destructive.  In the 19th century smoothing was perceived as an improvement.  Today it is seen as destructive but it is still done by unscrupulous "conservators" to deceptively "improve" coins.  

Smoothing is sometimes incorrectly used to describe a form of cleaning where a rough green patina or copper oxide encrustations (often red lumps) are smoothed down to field or detail level.  This "smoothing" is better described as cleaning and can enhance the appearance and value of a coin if it is limited to removing encrustations and protrusions.  To prevent confusion, the term smoothing should not be used to describe this type of cleaning.  The term smoothing should be used only when the metal of the coin has been altered.  

Some well done light smoothing is acceptable to many collectors, particularly on large bronze coins from old 19th century collections, with "old cabinet toning."  Sometimes even some tooling can be tolerated.  A collector may find a smoothed or tooled coin worth purchasing if the coin is a particularly desirable type or priced low.  Do not, however, pay more for a coin that has tooling or smoothing "improvements" than you would for a worn or pitted example.  Tooling and smoothing are damage and reduce rather than enhance value.  How much smoothing or tooling you find tolerable is a personal decision.  Many collectors will not tolerate any smoothing or tooling whatsoever.