means,
Active, efficient, mighty; producing the desired effect. Also, having the necessary power or force to produce the desired effect.

And the prayer of faith shall save the sick, and the Lord shall raise him up; and if he hath committed sins, they shall be forgiven him. Confess your faults one to another, and pray one for another, that ye may be healed. The effectual fervent prayer of a righteous man availeth much.
James 5:15-16