ilaplace with rootof question

Hi all,

I have a question with inverse laplace transform with my function.

My function is as follows:

(s^3 + (8323294838784001*s)/8192)/(s^4 + 1872229900000*s^2 + 844381359599999923519488)

after using the ilaplace transform on it, it becomes this:

sum((r15^2*exp(r15*t))/(4*(r15^2 + 936114950000)), r15 in RootOf(s15^4 + 1872229900000*s15^2 + 844381359599999923519488, s15)) + (8323294838784001*sum(exp(r15*t)/(4*(r15^2 + 936114950000)), r15 in RootOf(s15^4 + 1872229900000*s15^2 + 844381359599999923519488, s15)))/8192

I would like to ask what is the meaning of r15 in RootOf? I assume s15 is a variable that is declared in this answer.

any insights with regards to this?