The XMSS tree is constructed using the W-OTS keys. Each leaf of the XMSS tree corresponds to a distinct W-OTS key. The W-OTS keys have length $ln$, but each node of the XMSS tree should have length $n$.
In order to make the size of the W-OTS keys compatible with the XMSS tree, the L-tree is used to first reduce the W-OTS key from $ln$ to $n$ bits.The L-tree divides the W-OTS key into $l$ $n$-bit strings, which then form the $l$ leaves of the L-tree. The root of the L-tree is $n$ bits and can be used as a leaf in the XMSS tree.
Source: "XMSS – A Practical Forward Secure Signature Scheme based on
Minimal Security Assumptions" from Buchmann and Hülsing