This is the Thistlethwaite name of the link. It is of the form L[k]a[j1] or L[k]n[j1], depending on whether the link is alternating or not. Here, k is the crossing number and j1, j2 are archive numbers assigned to each (a, k), (n, k) pair, respectively. For example, L5a1 is the first alternating link with 5 crossings while L6n2 is the second non-alternating link with 6 crossings.
References
[1] Hoste, J., Thistlethwaite, M., and Weeks, J. The First 1,701,936 Knots, Math. Intelligencer, 20 (1998), 33-48.