Every molecule of H2SO4 contains two hydrogen atoms, so every molecule of H2SO4 is capable of forming two H+ ions. Remember that H2SO4 is a strong acid which dissociates fully into ions when dissolved in water.

By similar argument, we can say that 1 mole of H2SO4 is capable of producing 2 moles of H+ ions.

(mole is not exactly a shortened form of the word molecule, but the ratio idea still holds)

1 H2SO4 ==> 2 H+ ions
1 mol H2SO4 ==> 2 mol H+ ions

By this ratio,
0.025 mol H2SO4 ==> 0.05 mol H+ ions

So, 0.05 mole of H+ ions is contained in 25.0 cm3 of sulfuric acid.

This means that 0.05 / (25/1000) = 2.00 moles of H+ ions ae contained in every 1000 cm3 of solution.

So, the concentration of the hydrogen ions is 2.00 mol/dm3.

(In contrast, the concentration of the acid itself is only 1.00 mol/dm3)