I really do not understand, can you explain these grammar for me please?
We use the perfective will have when we are looking back from a point in time when something will have happened.

By the end of the decade scientists will have discovered a cure for influenza.
I will phone at six o’clock. He will have got home by then.

or looking "back" from the present:

Look at the time. The match will have started.
It’s half past five. Dad will have finished work.

We use would have as the past tense form of will have:

I phoned at six o’clock. I knew he would have got home by then.
It was half past five. Dad would have finished work.

Question

I really do not understand, can you explain these grammar for me please?
We use the perfective will have when we are looking back from a point in time when something will have happened.

By the end of the decade scientists will have discovered a cure for influenza.
I will phone at six o’clock. He will have got home by then.

or looking "back" from the present:

Look at the time. The match will have started.
It’s half past five. Dad will have finished work.

We use would have as the past tense form of will have:

I phoned at six o’clock. I knew he would have got home by then.
It was half past five. Dad would have finished work.

Answer

The tense you are talking about is called future perfect.

It involves a relation between two distinct times in the future:

1) Is an indeterminate time when an action occurs.

2) A more specific time after the action has already occurred.

Let's look at your first example. Both times mentioned are in the future but we only know specific details about one.

"By the end of the decade..." It will be around 2020, which is the end of this decade.

The second event, "the cure for influenza is discovered", we have no information regarding when it happens except that it happens BEFORE the end of the decade.

So, by the end of the century (which is in the future so we use the future tense "will") the discovery of a cure for influenza will be in the past. But because the date of this "future" past is unspecific we use the "present perfect" form "has been".

By the end of the decade the cure for influenza "will" "have been" discovered.

Notice that "has" becomes the simple present "have" because of the word "will".

So, future perfect = "will have" + "past participle".

Answer

In regards to the "looking back from the present", the best explanation I can give as to why we would use a future perfect in this case is this:

The indeterminate action has not been confirmed yet. It will be confirmed in the future.

"Look at the time. I bet that if I check, the match 'will have started'. But I don't know for certain yet because I haven't checked yet."