The whole formula means to find the definite integral of ic(ξ) in the integral interval (t0, t);

Ic(ξ) indicates that this is a function of ξ, where dξ indicates the integral of ξ,

ξ is an integer variable.

If you write dt, then t is an integer variable. The upper and lower lines of integral cannot be confused with integral variables. Specifically, you can check the concept of integral.