In the US, many forms of government cooperation and compulsion are public. Some can be secret, or secret for a period of time. The general idea is that they are public. A company or person may wish to keep such assistance to the government secret or confidential, but there is often no requirement from the government to do so. However, in recent years, that has been changing to a default "make all the things secret" philosophy, and the increased use of National Security Letters.
Warrant canaries are directed at National Security Letters (NSLs), which historically, from what little we know about them, also come with a permanent gag order (aka forced secrecy forever). Because NSLs are issued under the "National Security" umbrella and apparatus (link), the government says they fall outside the normal scope and rule of law, and as such you cannot talk about it or will get thrown in to Gitmo and they will throw away the key. Maybe not literally, but they threaten to do all sorts of horrible things and prevent even the receiving entity of an NSL from seeking counsel, which many view to be an abuse of power and due process.
The warrant canary is an attempt to be a solution for companies, who now hold all our private information (willing or unwittingly) to let their users know if they have been breached by the government by use of force or coercion. This is in addition to the statistics and breach reports companies regularly disclose to their users. The theory is that you can be compelled to be silent, but you can't be compelled to say something, or a particular thing. Therefore, the warrant canary will die when the company goes silent.
A particular warrant canary is only good for a particular length of time, and then must be replaced, refreshed, or updated. If it gets stale or disappears, presumably, an NSL was issued to the company, and that company has been forced to turn over data of one or more of its users.
Source: reading the news on this stuff.