Pages Fields - Disable/Enabling

I'm not sure if this has been brought up before or not - but how would one go about enabling/disabling fields depending on the selection of a previous field?

For instance, let's say you have three fields. The first field let's you choose one of two selections in a Select Box field.

You select the first selection in the Select Box Field, and the third field grey's out and/or becomes invisible/unseen. If you select the second selection in the Select Box Field, the second field grey's out and/or becomes invisible/unseen, while the third field remains visible or appears.

Any ideas on how to accomplish this? One might consider this "conditional fields", as an e.g.,: https://codepen.io/memco/pen/BajpD