I try to make sense of equation (3) in this paper:
$$S_W=\frac{\sum_{i=1}\sum_{j=1}^{J_i}(x_{ij}-\bar{x}_i)(x_{ij}-\bar{x}_i)'}{N-1}$$
"Equation 3 is used to compute the (pooled) within covariance matrix (SW)."
What does the prime symbol at the right side of the equation mean in this case? Does it simply describe the end of what is affected by the sigma? Or is it something that I need to address explicitly when translating this equation into code?