[trinhn812]

Someone told me H2O2+2e- --> H20, but what is that suppose to mean?

[azmanam]

There are a couple of half reactions H₂O₂ can partake in:

O₂(g) + 2e⁻ → H₂O₂(aq)
HO₂^• + e⁻ → H₂O₂(aq)
H₂O₂(aq) + 2e⁻ → 2H₂O

Given these UNBALANCED half reactions, the procedure used to balance them is to add water molecules and either protons (H⁺) or hydroxide ions (OH^-) to the equation, depending on whether the reaction is conducted in acidic or basic conditions.  see here for more information

http://www.chembuddy.com/?left=balancing-stoichiometry&right=half-reactions-method
http://www.chemguide.co.uk/inorganic/redox/equations.html
http://www.ausetute.com.au/halfeqtn.html
http://dbhs.wvusd.k12.ca.us/webdocs/Redox/Balance-HalfReactions-Base.html