The clientlib is a component of the Trustlines Protocol. The Trustlines Protocol is a set of rules to allow the transfer of value on top of existing trust relationships stored on a trustless infrastructure, here a blockchain.
In the technology stack, the clientlib is located on top of a relay server, and a mobile application can be built on top of it.
The goal of the clientlib is to provide a user focused means to interact with Trustlines via a relay server. It also provides a means to handle the wallet and key for the user. The intent is to make it as easy as possible to build a user interface for Trustlines on top of it.