Polities are groups of people who come together to make decisions and enact their will. The decisions they make collectively are agreements. An agreement is generally made about a document, which generally consists of a list of statements which the members of the polity generally agree to.
There's a lot of "generally" in there, because different polities do things in different ways.
Depending on the rules of the polity, different conditions may apply to how an agreement gets accepted. When an agreement has been accepted under the rules of a polity, it is normally said that it has been "adopted". In the most simple approach, if more than half of the members of the polity who participate in a vote on the matter agree, then the relevant document gets adopted.
More complicated methods might require consensus (everybody agrees), some other amount of support (say, 2/3), or there could be a minimum number of members required to cast votes in order for it to be adopted.
A more obscure, but interesting condition, is "rolling adoption" - that at every point in time, enough members of the polity must accept the document for it to remain adopted, otherwise it becomes "unadopted". This requires that new members go through previous agreements and acccept or reject them, otherwise they might simply be aged out of the polity.
Depending on the rules of the polity, an agreement document may have very varying structure. A common example for international treaties is that documents consist of references to previous agreements, followed by a list of assumptions made by the authors of the agreement, followed by a list of declarations which the signatory polities agree to. Here is a delightful example - hover over different parts to see what they're for.