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The total electric field or force from multiple charges is the vector sum of fields or forces from each charge individually. This linearity allows solving complex configurations by breaking them into simple pieces and combining results.
From Coulomb's law you know that a single point charge q₁ exerts a force on a test charge q₀ that depends on distance and direction. Now suppose there are two source charges, q₁ and q₂, both present simultaneously. Does q₁'s effect on q₀ change because q₂ is also in the room? Experimentally, the answer is no — each source charge acts entirely independently. The total force on q₀ is simply the vector sum of the force from q₁ and the force from q₂, as if each acted alone. This is the superposition principle: the interaction between any two charges is unaffected by all other charges.
This linearity is not logically necessary — it is an empirical fact about electrostatics that happens to follow from the linear structure of Maxwell's equations. Because the governing equations are linear (no terms where E² or E·B appear in Coulomb's law), any solution can be built by adding other solutions. This is profoundly useful: no matter how many source charges you have, you never need a new method. The answer is always "compute the contribution from each charge, then add the vectors."
In practice, superposition means you decompose a complicated charge distribution into pieces you can handle. For three point charges, you compute three Coulomb forces (or fields) and add their x-components together and their y-components together. For a continuous distribution — a charged rod, disk, or sphere — you mentally slice it into infinitesimal point charges dq, compute each dq's contribution dE as a vector, and integrate. The integral is just a continuous version of the same vector sum. Every calculation you will do for continuous charge distributions in electricity and magnetism rests entirely on this principle.
The most common error when applying superposition is treating the contributions as scalars rather than vectors. You must keep track of direction: a positive charge at position A pulls a test charge toward A, while a negative charge at position B pulls toward B. Those directions may partially cancel, giving a net field much weaker than either alone — or they may reinforce. Always draw the individual field vectors first, decompose them into components, then sum. The geometry is the hard part; the principle itself is simple: when sources are independent, results add.