It is from Crash Course Biology. It is at around 9 minute and 15 second. Here is the context:
And now for what is, totally objectively speaking of course, the coolest part of the animal cell: its power plants.
I am totally confused why the host used for. It would make sense to me if the word were omitted. Tell me please what its meaning there.
Speakers who are addressing audiences, especially in television and theatrical settings, frequently introduce new ideas with expressions such as:
And now for something different/amazing/incredible
And now for the big news
And now for the first look at our new car
The basic expression is and now for followed by whatever the speaker wishes to reveal. The idea is to prepare the audience for some interesting or exciting development.
In your example the speaker is saying the equivalent of:
And now for something that is the coolest part of the animal cell...
But instead of using the words something that, the speaker says:
And now for what is .... the coolest part of the animal cell
So the expression for what is is just a dramatic way of introducing something. It's equivalent to saying this is the coolest part of the animal cell.
The introductory three-word phrase "And now for..." is often used as an idiomatic, attention-grabbing expression to alert the audience that something significant about to happen.
For example, toward the conclusion of his act, a magician might say:
"And now for my final trick..."
The comedy troupe Monty Python would often interject:
"And now for something completely different."
as an transition between sketches; they even used that quote as a title for a compilation film.
One could argue that the for could often be omitted, but that might require a comma in the written version, or perhaps a brief pause in the spoken version:
"And now, my conclusion..." (vs. "And now for my conclusion...")
Sure, the speaker's sentence could be rearranged and shortened some:
Now, the coolest part of the animal cell (totally objectively speaking, of course) is its power plants.
but you've asked enough questions about these videos to know that speakers don't always word things in the most succinct way when speaking off-the-cuff.
