A provision is a legally binding clause in a contract or agreement. Provisions are often put into contracts or agreements in order to articulate the terms of the contract or agreement that is being made.
In the context of the law, each party in a contract or agreement is bound to the provisions of the contract if the agreement or contract is legally binding.
Often, businesses or individuals hire lawyers to draw up contracts. Part of the reason why they do this is because lawyers know how to write provisions into a contract in a legally optimal way.
Contracts are made for a wide variety of reasons. For example, one business may want to make a large order from another, a person may want to buy insurance from an insurance company, etc.
No matter what the contract is for, the provisions will help determine each party's legal responsibilities.