A bashed patch is a user created patch to fix inconsistencies in mods. It allows for leveled lists and changes to items to play well with each other. If you have multiple mods that add things to leveled lists, or things that edit the same things, you should create a bashed patch.

Is bashed patch necessary?

No, not 100% required - a bashed patch is a way to combine certain elements of various mods into a unified single mod ( the bashed patch is treated as a mod) that then overwrites all of those elements in the various mods they come from. This prevents those elements from interfering with each other while playing.

What is bashed patch Skyrim?

A bashed patch mainly merges leveled lists from various mods so that they play nice together. It may also merge a few mods into it so you will no longer need that mod's ESP (reducing the number of ESPs in your load order).