Often, unless you have an employment contract, you can be fired with relatively little reason. As long as they don't fire you because of your gender, race, age or another protected class, your boss has a lot of freedom to keep or terminate your employment.
If you do have a contract, that changes things significantly. Many contracts specify that the company has to show cause to terminate that contract, for instance, or they lay out a system of warnings and write-ups that have to be given out before someone's position can be terminated. This gives you far more protection.
But what if you have an implied contract? Even if you never technically sign a contract, the company could imply verbally or in writing that you have some guarantee of employment or that your job is protected.
For instance, if they hire you and say that they won't fire you for at least five years, even if they say it in passing, you could claim you thought that was an implied five-year contract if you get fired sooner. Some employees also point to the handbook or other documentation that they're given, saying that it is an implied contract as long as they follow what it says.
As such, many employers have a separate document that they make employees sign, saying that they understand that the documents are merely guidelines and do not offer any real protection. If you didn't have to do that, you may be able to claim it was an implied contract.
You can see that employment is not always as simple as it first appears, and you need to know all of your rights.