Em7b5 (E half-diminished seventh) is a four-note chord made from E, G, Bb and D. A half-diminished chord is a diminished triad with a minor seventh on top. It is the ii chord of a minor key, so it opens the minor ii-V-i that jazz leans on.
Built from the root E, the minor third G, the flat fifth Bb and the minor seventh D.
Frets from low E to high E; x means don't play that string.
Easiest shape to start with: Open position (0100xx). It has 3 open strings, nothing above fret 1 and 2 muted strings.
Every note of Em7b5 is in these keys. The numeral is its place in the key.