Ratification is a process through which an agreement is validated so that it becomes binding. Several types of agreements must go through a ratification process in order to be valid. The classic example is a treaty, but federations such as the United States also use ratification for Constitutional amendments, and ratification can come up in contract law as well. In contract law, when something is undertaken by an agent, it may need to be ratified before it will take effect.
With treaties, the negotiation of a treaty often involves agents who act on behalf of their parent nations. These agents work together to draft a document which they believe will be accepted by all of the nations involved, and then they return to their home nations with the treaty. The treaty must be ratified before nations will be bound by it. This is designed to ensure that nations have a chance to approve or repudiate the actions of their agents, and that countries do not become trapped into treaties they do not wish to endorse.
Treaty ratification is usually done through an act of government in which the legislature votes on whether or not to ratify the treaty. If the vote passes, the treaty is signed and approved. Often treaties are structured with a trigger clause so that they enter into force when signed by a set percentage of the parties. This is why some treaties are said to be ratified and in effect even though they have not been signed by all of the parties yet.
For amendments in nations which permit them, usually the legislature must approve by a majority. In addition, there may be other laws in place, like requirements that individual states or provinces also have to approve, or that successive majorities in the legislature must approve. It is also possible in some regions for an initiative developed by citizens to be used to amend the constitution.
In contract law, some contracts have to be ratified before they are valid. Ratification is usually required when contracts are negotiated on behalf of someone else. In a simple example, when unions bargain with their employers, they return to the union members with a contract and they must vote on it. If it is rejected by the union members, it is not ratified and the union must return to the negotiating table to attempt to develop a contract which will be accepted.