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A signal with a value that can take any value, typically between two limits. We say such values are ‘continuous’, i.e. there are no intervals or steps. An example is shown below. Here a signal $v_s(t)$ represents a voltage that is changing with time $t$. The signal can take any value such that $V_L \leq V_s(t) \leq V_U$, where $V_L$ and $V_U$ are the lower and upper limits.
All electrical systems are ultimately analogue when you look closely enough. I contrast, digital systems employ high-gain amplifiers to switch between two extremes very quickly, giving the impression of working in only two states.