what does the following element description actually mean? iron-57

 the following description mean that the nuclide is an isotope of the atom of iron,  it has atomic number of 26 and consequently a neutron number of  57-26 = 31.

 iron -57 is an isotope , isotopes has same atomic number but different mass number because they unequal number of neutrons,

all isotopes of an atom behave same way chemically, but only the nuclear proper, mass and density that varies.