It means there is room for invincible ignorance, period. Salvation is through the Church alone, which in turn is the mystical body of Christ. Those who, through no fault of their own, do not know the Gospel of Christ or his Church, but who nevertheless seek God with a sincere heart, and, moved by grace, try in their actions to do his will as they know it through the dictates of their conscience - those too may achieve eternal salvation. For those who are not formally and visibly members of the Church, salvation in Christ is accessible by virtue of a grace which, while having a mysterious relationship to the Church, does not make them formally part of the Church, but enlightens them in a way which is accommodated to their spiritual and material situation.
To go beyond this and claim that anything other than Christ can bring salvation is just erroneous.