An Input is something you use to carry out an Activity and an Output is something produced by an Activity.
An Input is something you use in order to carry out an Activity. For example, in the process making a cup of tea, if the Activity is boiling water in the kettle, the input is water.
An Output is something you produce as a result of carrying out an Activity. For example, in the process making a cup of tea, if the Activity is boiling water in the kettle, the output is boiling water.