That is another word for the wave function ψ. It is called the probability amplitude because the square of its absolute value is the probability density W = |ψ|2. This probability W tells US WHETHER the quantum particle is likely to be in a certain STATE or location. If W is large then the probability is high. If W approaches zero then the probability approaches zero. If we integrate the probability density over all all space, the total probability must be 1. A GOOD reference discussing the wave function and wave packets etc.

That is another word for the wave function ψ. It is called the probability amplitude because the square of its absolute value is the probability density W = |ψ|2. This probability W tells us whether the quantum particle is likely to be in a certain state or location. If W is large then the probability is high. If W approaches zero then the probability approaches zero. If we integrate the probability density over all all space, the total probability must be 1. A good reference discussing the wave function and wave packets etc.