well, before explain what PRESENT PERFECT is, it is best to remember SIMPLE PAST.
Look at these sentences:
I WENT TO THE MOVIES YESTERDAY.
I ATE OUT WITH MY BOYFRIEND LAST NIGHT.
I SAW THE NEW TARANTINO MOVIE LAST WEEKEND.
All these three sentences are in SIMPLE PAST and they happen in a particular time in past (YESTERDAY, LAST NIGHT, LAST WEEKEND...)
And that is how SIMPLE PAST is used, to describe an action that happened in a particular time in past, because time is important in this case.
PRESENT PERFECT describes an action that happens in past too. Take a look at these sentences:
I HAVE LIVED IN TORONTO FOR FIVE YEARS.
MY MOTHER HAS CLEANED THE HOUSE TWICE THIS WEEK.
In this case, TIME is not important but the action itself.
When I say that "i have lived in toronto for five years" it means that five
years ago i moved to Toronto and I am still living in Toronto today.
So PRESENT PERFECT can be used to describe an action that happened in past but it is not finished yet.
You can use PRESENT PERFECT to describe an action that happens during a period of time that is not over yet (this week, today, this month, lately) as you can see in the second example.
"My mother has cleaned the house twice this week." Today is Monday, the week is not over and my mother cleaned the house. She cleaned the house twice!