Etymologically speaking, monotheism is a word that is made up of two Greek words, meaning "one" and "god." It has come to refer to a kind of religion that believes in the existence of only one god and is the opposite in this sense of polytheism, which refers to a religion that has many gods, such as Hinduism. The main monotheistic religions of today are of course Judaism, Christianity and Islam.
Hebrew monotheism therefore refers to the way in which, as narrated in the Old Testament, and in particular the Pentateuch (the first five books of the Old Testament consisting of Genesis, Exodus, Leviticus, Numbers and Deuteronomy), the Hebrew religion moved away from polytheism and became the first monotheistic religion. This of course was enshrined in the Ten Commandments, the first one of which clearly delineates the way in which the Hebrew religion differed sharply from the religions of other tribes and cultures through its firm and strident belief in the existence of one God alone that was acknowledged as the true god. In addition, you might like to consider how the book of Genesis and its account of creation both is similar to and different from other creation narratives of the time. One of the central differences is the way in which Genesis acknowledges the presence of one God who deliberately makes humans to be in relationship with him, as opposed to many gods who fight over earth and humans. Again and again, the overwhelming message of the first five books of the Bible is one of monotheism.