A maintenance page is a page which informs visitors to a website that the website is undergoing maintenance and is not available. All of the content on the website still exists, but it is not accessible to visitors during the duration of the maintenance. People with authorized access to the site may be able to see and manipulate the website, allowing them to perform necessary maintenance.
There are several reasons to use a maintenance page. In some cases, maintenance will irritate users and destabilize the website, making it easier to take the website down while maintenance is completed to avoid frustrating people. During maintenance the look of the site may change rapidly, links could change, and other abnormalities could occur; people on the site could lose work, get confused, or accidentally interfere with the maintenance process. In these instances, putting the site into maintenance mode will make it easier for the person or people doing the maintenance to focus on implementing changes.
In other cases, the entire site actually needs to be taken down for work, in which case a maintenance page is necessary to alert people to what is going on. A website may be changing content management systems, implementing substantial upgrades, or taking other steps to improve performance which could result in temporary inaccessibility.
Many website maintenance systems and content management systems come with a basic maintenance page template. The template usually allows users to change the maintenance message; the message could direct visitors to another temporary site, provide information about when the upgrades should be completed, and so forth. It is helpful for visitors to have information like "the site is down for maintenance between 2:00 AM UTC and 6:00 AM UTC" so that they know when to check back.
Some websites undergo regularly scheduled maintenance. In this case, warnings may be posted several days before the maintenance to let users know that the site will be inaccessible for a set period on a certain day. Especially for things like online banking, people usually like to know in advance when they will not be able to access a website, so that they can plan around it.
Sometimes maintenance takes longer than expected, in which case it is a good idea to update the information on the maintenance page to keep people informed. Some websites may also offer to update readers by email when the site is available again.