What is a leader of the opposition?
When there is a Parliamentary election, the party that wins the
most seats, or that put together a coalition having the most seats,
will form the government, and the party which comes in second will
form the opposition. The reason it is called the opposition is
because it exists to offer an alternative to the ruling party. If
it should turn out that the voters are not pleased with the job
that the ruling party is doing, they can turn to this other party
and elect them instead.
All parties have leaders, and so, the opposition party has one,
who is known as the leader of the oppostion. Such a person aspires
someday to become the Prime Minister (which may or may not ever
actually happen).