The "group operation" refers to the operation that combines two elements of a group to produce a third element within the same group. In the context of the special linear group $\operatorname{SL}(n, F)$, the group operation is matrix multiplication. This means that if you take two matrices $A$ and $B$ in $\operatorname{SL}(n, F)$, their product $AB$ (resulting from matrix multiplication) is also in $\operatorname{SL}(n, F)$ and will have a determinant equal to 1.