Discrete Slant Stack on seismic data
The $\tau$–$p$ time domain $t$ is a function of offset $d$, so the result is an array of size $n_t \times n_d$, where $n$ is the number of samples.

The inputs are a vector $\tau$ of travel time at zero offset, or $t(0)$, an array $p$ of ray parameter, which is in turn a function of velocity $V$ and angle, and a vector $d$ (usually called $x$) of offset.

Here's an example of some visualizations of the $\tau$–$p$ domain from a paper by Sergey Fomel; they might help explain what I mean by an 'array' — you can think of them as images:



The resulting travel-time $t(d)$ is an array of size $n_t \times n_d$, where $n$ is the number of samples. For a given offset at a given time sample, the result is a scalar — a single value of $t$.

By the way, stevej's answer is a great explanation of how exactly to calculate the ray parameter term, given limited information. Do follow that link to Yilmaz, and maybe also read the Radon transform article.