When the HTTP specification talks about “safe” HTTP methods it means that no resource should be destroyed. The GET method is a safe HTTP method because it doesn't change the resource. It only retrieves it and that's it! On the other hand a POST method is an unsafe HTTP method because it changes the resource on the server, e. g. updates an account, submits an order, etc.
This browser behaves differently when executing GET or POST methods. For example you can easily refresh a webpage that was retrieved via a GET method, because the browser would just render the same HTTP response that he would get from the server just as before. However if you want to refresh a webpage that was retrieved via a POST method, then we would get a warning.
Therefore web applications try to show the user a GET retrieved webpage only by following the so called POST/Redirect/GET pattern:
If the user clicks a button to POST a request (e. g. submitting an order), then this request will be sent to the server. The server will then reply with an HTTP redirect, meaning the server tells the browser to GET another resource at a specifed location. The browser will follow this command and will GET the resource. The server will then reply with for example “thank you for the order” resource. By doing so the user can now refresh the webpage, since it is now a resource that was GET instead of POST.
Source(s):
HTTP Succinctly by Scott Allen Syncfusion
Wikipedia
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