Can I improve the idea of the paper on arxiv and publish it again? I have an idea, and I did experiments and then wrote a paper. But the conference I want to submit does not start until a month later. I want to upload it to arxiv first. But I found a description of the shortcomings of arxiv on Google. Others may scoop papers by improving the papers uploaded to arxiv.
If others see my results. Then immediately embark on the same experiment. Then change the description method (note that the ideas in some papers are very new, but once thought of it, it is easy to reproduce). Or they use the same idea to do some other evaluations (such as changing the background). If this is the case, won't my efforts be wasted?

Authors are not barred from uploading their papers to arxiv and
similar sites. But please note that such ef- forts may compromise the
double-blind review process, so please exercise care when discussing
your submission in public forums. On a related note, unrefereed
on-line pre-prints are not assumed to constitute “prior work” – in
other words, reviewers cannot penalize an HPCA submission because it
does not cite a pre-print with lim- ited visibility.

This is a passage from the conference where I want to contribute. It can be seen that he is not opposed to uploading the paper to arxiv.
But what does the last paragraph of it mean?

On a related note, unrefereed on-line pre-prints are not assumed to
constitute “prior work” – in other words, reviewers cannot penalize an
HPCA submission because it does not cite a pre-print with limited
visibility.

Does this sentence mean that it doesn't matter if you plagiarize a paper on arxiv. Because they don’t think arxiv is “prior work”.
arXiv is a public repository/a preprint server for research papers (that are either under review or currently being worked on. There are other such servers hosted by publishing houses as well like Elsevier's SSRN or MDPI's preprints.
In accordance with your question to the following paragraph:

On a related note, unrefereed on-line pre-prints are not assumed to constitute “prior work” – in other words, reviewers cannot penalize an HPCA submission because it does not cite a pre-print with limited visibility.

What they are trying to convey is that, if a paper (an online pre-print i.e., present on any of the public repos/databases) is somewhat relevant to the topic you're targeting in your paper and you don't cite the paper in the related work/prior work section you won't be penalized for it or asked to do so. The reason they don't consider arXiv prior work is clear that the PC is strictly adhering to papers that are rigorously peer-reviewed and published by a journal.
Also, it clearly does not indicate that you can plagiarize anyone's work.