In their comment, Sylwit pretty much explains what this has to do with search engines. I'm going to just describe the differences between GET and POST
GET and POST are two different types of requests to the server. A GET request is normally used to retrieve information from the server and usually has a series of GET parameters. When you search something on Google you're making a GET request.
https://google.com/?q="how+do+i+get"
In this case, the GET parameter is the q after the ?, and has a value of "how do i get". It should be noted that a GET request doesn't need these additional parameters (http://google.com) is still a GET request
POST requests, on the other hand, are normally used to send data to the server. You'll see this anytime you send a message, submit a form etc. When I click submit on this answer, I'll be making a POST request to stackoverflow's servers. The parameters for these aren't immediately visible in the browser. POST requests can also return a HTTP response, with a message.
Hope that shows the differences between the two.