There are several types of seams, which can be made with a single needle (i.e., one sewing thread) or a double needle (i.e., two sewing threads), for example. 

A seam made with a single needle will be less resistant than a seam made with a double needle. This is what we mean by "the weakest seam." 

This is the seam that is most likely to fail to hold the fabric in place and eventually tear.