# What does it mean to “concentrate” a function with respect to a quantity?

I was reading a text about linear models and regression and came across the statement:

"To find the maximum likelihood estimates for $\theta$ and $\sigma^2$ the log-likelihood must be concentrated with respect to $\sigma^2$." [1]

How does one "concentrate" a function with respect to a some quantity? I don't understand what operation is being referred to here.

[1] "Linear Models and Regression." Pharmacokinetic-Pharmacodynamic Modeling and Simulation, by Peter L. Bonate, 2nd ed., Springer, 2014, p. 63.