In every economy, some goods are provided by the government to the entire people. Such goods are called public good. Specifically, public good is the one that is provided to the society as a whole and consumption by one individual doesn’t reduces its availability or doesn’t exclude others from consuming it.
So there are two important features for public goods- non rivalry (doesn’t reduce availability for A if B consume it) and non-excludability (no one is excluded from consumption).
Example for public goods are national defense, public parks, street lights, and other basic societal goods.
The expense for providing public goods are met by the government out of taxes. This means that an individual (with the ability to pay taxes) who avoids or evades taxes enjoy public goods and he became a ‘free rider’.
Free rider problem says that a rational person will not contribute to the provision of public good because he does not need to contribute to the benefit from it.