This formula describes a transformation where a vector, denoted as $\mathbf{k}_i$, is mapped to a new vector, $\mathbf{k}'_i$, by applying a function $\phi$. The bold notation indicates these are vector quantities. The subscript $i$ suggests this operation is performed on an individual element within a set or sequence of vectors. In the context of Large Language Models, this can represent the transformation of key vectors within an attention mechanism.