The High German languages (in German, Hochdeutsch) are any of the varieties of standard German, Luxembourgish and Yiddish, as well as the local German dialects spoken in central and southern Germany, Austria, Liechtenstein, Switzerland, Luxembourg and in neighbouring portions of Belgium, France (Alsace and northern Lorraine), Italy, and Poland. The language is also spoken in diaspora in Romania (Transylvania), Russia, the United States, Argentina, Chile, and Namibia.
As a technical term, the "high" in High German is a geographical reference to where the dialect family that forms High German originates. It refers to the mountainous areas of central and southern Germany and the Alps. This is opposed to Low German, which is spoken along the flat sea coasts of the north. High German can be subdivided into Upper German and Central German (Oberdeutsch, Mitteldeutsch).
Please note that there is a difference between how linguists use the term Hochdeutsch and how German popular culture uses the term. The average German speaker will always assume that the term Hochdeutsch refers to standard German as opposed to dialect, and not to an entire linguistic branch of the German language. This is possibly due to a folk etymology that interprets the term hoch in the sense of "higher" in a cultural sense, i.e. as the "elevated" way of speaking. In English discourse, on the other hand, the term "High German" is never used to mean "Standard German."