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One qubit could hold 0 and1 simultaneously and perform some operations/calculations on it simultaneously.

Two (entangled) qubits could hold 00, 01, 10, 11 simultaneously and perform calculations on these four values simultaneouslty.

...

100 (entangled) qubits could hold 2^100 distinct values simultaneously and perform calculations on them simultaneously.

That's the basic idea. The problem is keeping entangled state that's becomes harder and harder as number of entangled qubits grows.

Also there are limitations in extracting of calculations results from system of qubits, so not all algorithms can be speed up on quantum computers (but e.g. integer factorization can).