Hydrogen has a great affinity for oxygen. They
combine together to form water H2O, and
in the process a large amount of energy is
released. There are two interesting aspects in the
experiment of making explode the hydrogen balloon
that I would like to call your attention to.
First, that pure hydrogen will not explode
when ignited, it just burns with a quiet flame, no
explosion. The explosion occurs when hydrogen
has been previously mixed with oxygen (for
example, because oxygen can diffuse from outside
into the balloon). There is a range of mixture
that will explode (something like between 6 and 60
% of hydrogen, but these numbers I am quoting from
memory, if you need the exact range I would have
to look them up).
Second: You can mix hydrogen and oxygen and
they will stay together, apparently without
reacting forever. Without some initial push (given
by the flame of a match, for example), their
reaction is too slow to be perceived. But if we
give enough energy to get started, it will become
an extremely fast reaction: an explosion.
Hope this satisfy your question, if you still
have doubts, let me know.