What is the meaning of addition of charges ?

Electric charge is a scalar quantity.

The total charge of the system is obtained by adding charges algebraically.

If a system contains $n$ charges $q_{1}, q_{2}, q_{3}, \ldots, q_{n}$ then the total charge of the system is $\mathrm{Q}=q_{1}+q_{2}$ $+q_{3}+\ldots+q_{n}$

$\therefore \mathrm{Q}=\sum_{i=1}^{n} q_{i}$ where, $i=1,2,3, \ldots, n$

Proper signs have to be used while adding the charges in a system. For example, the total charge of Welcome To Future-Quantum Paper

a system containing five charges $+1,+2,-3,+4$ and $-5$ is some arbitrary unit is $(+1)+(+2)+(-$ $3)+(+4)+(-5)=-1$ in the same unit.