Elements and Substances

Nov 2018
Los Angeles
Hey guys, I am new to chemistry and I am having a lot of trouble understanding the what elements and substances are. In the IUPAC Gold Book, an element is defined as a species of atom defined by the number of protons, but it also says that an element is a substance composed of the same type of atoms. How are these two definitions of an element different? I have seen a chemical substance being defined as "a form of matter having constant chemical composition and characteristic properties" although I have absolutely no idea what this means. If someone could explain these things to me I would be very appreciative. Thanks a lot in advance.