Kettling, AKA containment and corralling, is a crowd control tactic in which officers form a cordon around a specific area, either trapping a portion of the crowd, or leaving only one opening through which people can exit.
The former case may be used to isolate a particularly rowdy segment of a crowd from calmer masses, to aid in arrests of targeted individuals, or in some cases, to deny protesters food and other necessities in order to better coerce compliance.
In the latter case, the goal is to have a controlled exit from the crowd that is guided, or monitored, by the police or other crowd control agents.
Since the mid-1990s, it has become increasingly common for police to use kettles to informally detain large groups of protesters, often for hours. While the stated use of kettling may be to aid in controlled crowd dispersal, it is all too easy to keep the kettle locked up for a sustained length of time before releasing members of the crowd. As kettling is framed as 'crowd control' rather than detention, it is easy to abuse in this manner. There have been many cases of bystanders caught in a prolonged kettle, and of kettles maintained beyond a reasonable length of time.
As of this writing, kettling is still legal in the USA, the UK, Canada, and most of Europe. However, it has, over time, become more common to see successful legal action against kettling abuses.