This poem is seemingly straight forward. The speaker, feeling restless, happens to see the dictionary opened to the "L" section and he sees the word "lanyard." This makes him recall the time he made a lanyard for his mother at summer camp. He notes that this one word taps into his memory more than any "cookie nibbled by a French novelist." This could be a reference to Marcel Proust's A Remembrance of Things Past.
The speaker describes braiding a red and white lanyard. Then he notes all the things his mother has given to him and taught him: life, how to walk, how to swim, etc. He notes that it would be obvious to say that you "can never repay your mother." And he goes further to conclude that, as children, we actually believe that our practically useless gifts make things even between mother and child. What is missing from this conclusion is the suggestion that, for benevolent mothers, it is not about "getting even" or settling up. Collins wisely leaves that up in the air, as to avoid further cliches such as "it's the thought that counts."
We could consider the lanyard as a metaphor of that "thought that counts." The lanyard represents all of the crafts and gifts a child gives to his mother. It is the child's way of "repaying" his mother (using "repay" within the context of "making us even.") The suggestion is that the child repays his mother with crafts, small gestures, and the kinds of gifts that many children make for their parents at summer camps.
Consider that the lanyard itself is a representation and a metaphor for the way the child tries to repay his mother. The lanyard is braided in red and white. In terms of imagery, there is a connection between these colors and the colors of the mother's gifts to the child:
Here is a breathing body and a beating heart,
strong legs, bones and teeth,
and two clear eyes to read the world, she whispered,
The red and white of the lanyard could be interpreted to represent the life the mother gave to the child: the red heart/blood and the white bones. With two clear eyes, the speaker would be able to "read the world" and perhaps notice the symbolism of images such as these.