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ELI5: What does it mean when a state makes reservations, declarations or understandings when it ratifies a treaty?

I was reading on how the US ratified the Convention on the Prevention and Punishment of Crime of Genocide while making two reservations, five understandings and one declaration. Can anyone explain what does it mean by reservation, understanding and declaration during ratification of a treaty, etc.? Thank you!
gpt
These are all basically stipulations on how the state will follow the treaty it has ratified. Different treaties will have different rules on how such tools should be treated, but such stipulations are usually respected. 

Reservations are the state modifying the treaty, and their obligations under it. They can be a simple: "We aren't accepting this bit and that bit" or something else. There are usually limitations to what can and can not be reserved from--usually that the reservation can not be "incompatible with the object and purpose of the treaty"--and the US has been roundly criticised for its reservations under the Genocide Convention. 

Declarations and understandings are basically statements of how the state will interpret the treaty. The state basically says that as to its understanding, this provision of the treaty means [X]. 

The document you'd want to consult when interpreting treaties is the Vienna Convention on the Law of Treaties, which is--yes--the treaty on treaties. The Vienna Convention has lots of rules on how treaties should be interpreted, and--although many states have not ratified it--it is widely respected, and often turned to.