If something is on tap it is freely or immediately available, like water from a tap .
Of course, in many countries of the world water is definitely not always available 'on tap' so this idiom would not be relevant to people from these countries.
Examples of use:
1. Information is always on tap on the internet.
2. Drugs are available on tap in some major cities in the world.
3. The football team has many substitutes on tap.
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