What is the lower bottom number the periodic table mean again?

The number above the symbol is the atomic mass (or atomic weight). This is the total number of protons and neutrons in an atom. The number below the symbol is the atomic number and this reflects the number of protons in the nucleus of each element's atom. Every element has a unique atomic number.

Explanation:

Atomic number means the total number of protons present in an element. On the other hand, atomic mass means the sum of total number of protons and neutrons present in an atom.

In a periodic table, number at the top of symbol of each element represents atomic number. On the other hand, lower bottom number represents the atomic mass of respective element in each block of the periodic table.

Hence, we can conclude that atomic mass is the lower bottom number in the periodic table.