Means literally “at first sight” or in other words “basically”
A lawyer will often say that a client has a good “prima facie” case. This means that after hearing a rough summary of what the client say, he thinks that he has a case which he is prepared to take on that has “legs”, or there are facts which warrant further enquiry and investigation.
After thorough investigation, it could turn out that there is no case because of further evidence that may be presented, discovered, or disclosed, in particular, by the Defendant.