When a judge issues a ruling stating that certain information can't be posted online or printed in newspapers (or sometimes, even discussed outside the court room), it's called a gag order.
The judge in a legal case often issues a gag order when she's concerned that knowing details about the case before it begins might influence potential members of a jury. The danger is that jurors may come to a trial having already heard or read details about it and make their decisions based on rumors rather than the facts they learn in court. Gag orders are one way to help ensure a fair trial.