Why numbers less than 25 are not prime

why we jumped from 5 to 25 and why we are sure that elements less than 25 will definetely not be a prime

Which part of the video are you referring to? Please mention the timestamp.

I cant understand from 10:50

It does not mean that numbers less than 25 are not prime.
We are skipping those for 5 because we have already check for those numbers before we reached 5.
Please watch the video again and work with some examples for more clarity.