An indictment is the formal way of accusing someone of a crime, usually a felony. A "sealed" indictment is just a fancy way of saying that it has not been made public. Once it is made public, it is then considered to be "unsealed" (I have not personally heard the term "open", but I assume it is what you are referring to, and may be a term used in your jurisdiction). The government obtains an indictment by presenting the evidence it has to a grand jury, which is made up of citizens from the jurisdiction. It is a secret process and jurors and the government attorneys cannot share the information that is obtained during that process with the public. During this time, it is considered to be "sealed". Once it gets the indictment, the government can maintain an indictment sealed for various reasons. For instance, it may have an ongoing investigation and does not want to make it publicly known for some reason (they are using a person/defendant who is working as an informant with the government, for instance).
Since an indictment is the formal way to charge someone with a felony, it must be read to the defendant in open court. Therefore it is public and is "open" at the time that it is done. Therefore, a person cannot go to trial on a sealed indictment, since trials are also public. So in answer to the second part of your question, an indictment is sealed when it starts, but will become unsealed (open) prior to the person receiving the charges formally (what is called the arraignment). In our system, a person would not go to trial on a sealed indictment
