I was confused when I understand the function of a MySQL

I was just confused and did not understand its function when calling variables A, B, C or D in calling MYSQL data.
Can you explain this to me about its function when calling data?
Thank you for those of you who are willing to answer basic questions from me because I just started to make a project

calling the code as below

$sql .= " JOIN detail_penilaian b ON a.id_penilaian = b.id_penilaian";
        $sql .= " JOIN pertanyaan c ON b.id_pertanyaan = c.id_pertanyaan";
        $sql .= " JOIN kategori_penilaian_kinerja d ON c.id_kategori = d.id_kategori";
        $sql .= " WHERE a.nik = '$nik' AND d.keterangan = $ha AND a.id_periode = $idper";