Can someone explain the following code? I need to learn what each part means so I can turn it into enemy movement in a space shoot-em-up

Vec2d playerPos;
Vec2d direction; // always normalized
float velocity;

I get the above is naming two 2d Vector objects, and creating a variable called velocity. I'm not sure what the normalized comment is about, though.

update()
{
   direction = normalize(playerPos - enemyPos);
   playerPos = playerPos + direction * velocity;
}