They are the only elements with only 1 occupied electron shell.
First, you need to understand how elements are arranged in a Periodic Table (they are not arranged randomly)
Elements are arranged:
in groups (vertical column) by their valence electrons number.
in periods (horizontal row) by their number of shells occupied by electrons.
For your question,
First row of periodic table consists of only elements which have only 1 shell
s orbital can only hold 2 electrons at maximum.
Only 2 elements fulfill this requirement/property:
H, Hydrogen (Proton number 1)
He, Helium (Proton number 2)
In neutral state (not charged), an atom proton number = electron number