How it works (simplified)
An ordinary bit is 0 or 1. A qubit can be in a superposition of both, and qubits can be entangled. A quantum algorithm uses interference so that right answers are amplified and wrong ones cancel out.
Why we don't have them at home
Qubits are extremely sensitive. They must be cooled close to absolute zero and lose information at the slightest disturbance. The biggest challenge is error correction: building one reliable logical qubit out of many physical ones.
A threat to encryption
A large enough quantum computer could break RSA encryption, which protects the internet, using Shor's algorithm. That is why the US NIST published the first post-quantum cryptography standards in 2024, and companies are rolling them out. More in Encryption and HTTPS.