Basically, the fractional coordinates show the fraction of each unit cell vector that contributes to the position of that atom.

  x_cart = A_x * frac_a   + B_x * frac_b + C_x * frac_c
  y_cart = A_y * frac_a  + B_y * frac_b + C_y * frac_c
  z_cart = A_z * frac_a  + B_z * frac_b + C_z * frac_c

where A, B, and C are the unit cell vectors and A_x is the x-coordinate of unit cell vector A.

POSCAR的晶胞矢量A，B，C乘以分数坐标的和。