What is "Timebase" ?

I'm always fascinated by how many people insist on answering a question when they clearly don't know the answer.

This is for so-called "off-speed" recordings. If you, for example, record at 100 frames per second with a 24 frame time base, that is very different from recording both with the same frame rate. That determines whether the clip from the example comes in as slow motion or plays back in real time and can be slowed down in post as needed with the 100 frames as your base.

IOW: The recording frame rate is the number of frames per second (FPS) that are recorded. The recording frame rate is different from the project time base not framerate, which is the rate at which the footage will be played back. The maximum frame rate for each format is determined by Project Time Base and Format and, depending on the format, can be baked in or not.

If you haven't recorded any such clips than the setting is irrelevant. You would use it to reverse the off speed effect upon import if you wanted to.

The framerate the timeline is set.

Those numbers are all framerates.

Guessing you are just starting out with video editing?

Start here https://helpx.adobe.com/premiere-pro/user-guide.html as this basic video editing.

Beware Premiere Pro has a steeeeeeeeep learning curve.

You need to do some homework wiki:

https://en.wiktionary.org/wiki/timebase#English

https://en.wikipedia.org/wiki/Frame_rate