What exactly does URLConnection.setDoOutput() affect?

There's setDoOutput() in URLConnection. According to documentation I should

Set the DoOutput flag to true if you intend to use the URL connection for output, false if not.

Now I'm facing exactly this problem - the Java runtime converts the request to POST once setDoOutput(true) is called and the server only responds to GET requests. I want to understand what happens if I remove that setDoOutput(true) from the code.

What exactly will this affect? Suppose I set it to false - what can I do now and what can't I do now? Will I be able to perform GET requests? What is "output" in context of this method?

You need to set it to true if you want to send (output) a request body, for example with POST or PUT requests. With GET, you do not usually send a body, so you do not need it.

Sending the request body itself is done via the connection's output stream:

conn.getOutputStream().write(someBytes);

setDoOutput(true) is used for POST and PUT requests. If it is false then it is for using GET requests.

Adding a comment, if you have a long lasting connection and you send both GETs and POSTs, this is what I do:

if (doGet) {    // some boolean
    con.setDoOutput(false); // reset any previous setting, if con is long lasting
    con.setRequestMethod("GET");
}
else {
    con.setDoOutput(true);  // reset any previous setting, if con is long lasting
    con.setRequestMethod("POST");
}

And to avoid making the connection long lasting, close it each time.

if (doClose)    // some boolean
    con.setRequestProperty("Connection", "close");

con.connect();              // force connect request