Snippets are script templates. In postman we can create scripts in multiple locations such as collection level, folder level and request level. Scripting can be done as pre-request scripts and tests.
Collection level scripts
Click on more option on the collection -> Edit
Same as we can add folder level scripts as pre-request scripts or tests in folder level by clicking more options on folder -> Edit and in request level by by clicking on more options request -> Edit
And in the right side of the window we can see quick scripts such as to get variables and set environments in different levels.