What is mole?

usually mol in chemistry

about 6.022140857 * 10^23
of anything
Just like a "dozen" eggs is 12
of them.

6.022140857 * 10^23 is called "Avagadro's number".
Google that for history.

The significance is that 6.022140857 * 10^23 atoms of anything has the atomic mass in grams. Like 6.022140857 * 10^23 Hydrogen atoms has mass of ONE gram.
similarly 6.022140857 * 10^23 H2 MOLECULES therefore has mass of TWO grams.

6.022140857 * 10^23 O atoms , a mol, has mass of 16 g
6.022140857 * 10^23 O2 molecules a mol , has mass of 32 g. etc