How does a stream cipher work?

A stream cipher works roughly like a one-time pad.
Just as in one-time pad, it can be broken if (A) you have insight into what the pad looks like, or if (B) the pad is used multiple times.
The difference is that the “pad” in a stream cipher is not random, but pseudorandom - called a “keystream”.
The pad/keystream essentially comes from a PRNG, which is seeded by (secret key + nonce).
So the PRNG seed has a constant secret half (the secret key), and a changing public half (the nonce).
The constant secret half (the key) prevents the first kind of breakage (A: attacker having insight into the pad).
The changing public half (the nonce) prevents the second kind of breakage (B: the pad being used multiple times).

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