The definite integral is to add up the physical quantities of each point on the curve.
Dl is one of the segments when L is divided into infinite segments. E is a function of l, and the position value of dl segment (infinitely small in length) is brought into E to get a numerical value ... ∫ means adding the E values corresponding to an infinite number of segments (all these).
E in the above figure is the electric field, and L is the boundary of a closed whole area around the electric field.
It means circular integral along this boundary. This is a form of definite integral. It's just that the starting point and the ending point coincide.
Specifically, this integral formula means that in the electric field E, the sum of the work done by a charge qo along the closed loop L should be 0.
Because: regardless of the route of charge and the distribution of E, q0 finally returns to the starting point, that is, the total displacement is 0, so the total work is 0.