The following is the mathematical definition of a Artificial Neuron,
$$\textbf{Activation function } f: \qquad y = f(w^t x) = f\left( \sum_{i=1}^n w_i x_i \right)$$
Given that $W$ is a vector, what does the symbol $t$ represent?
What operation does it represent on a vector $w$?
The $t$ stands for transpose. The result of the matrix multiplication between $w^t$ and $x$ is then simply the scalar/dot product between $w$ and $x$, which in turn is just $\sum_{i=1}^n w_ix_i$:
$$w^tx = \begin{pmatrix} w_1\\ w_2\\ \vdots\\ w_n\end{pmatrix}^t\begin{pmatrix} x_1\\ x_2\\ \vdots\\ x_n\end{pmatrix} = \begin{pmatrix} w_1 \; w_2 \; \ldots \;w_n \end{pmatrix}\begin{pmatrix} x_1\\ x_2\\ \vdots\\ x_n\end{pmatrix} = w \cdot n = \sum_{i=1}^n w_ix_i$$