Each House describes an area of your life. It is not necessary to have a planet in a house and indeed it is impossible to have all houses with planets in them (12 houses 10 planets).
Each house is ruled by a planet, the planet which rules the sign on the cusp of the house. Thus if your fifth house has Leo on the cusp it is ruled by the Sun and Solar energy will be exhibited in the way in which you approach 5th House matters, such as children, romance, sport and leisure activities.
If there is a planet in the fifth, then this will modify the way in which you approach 5th house activities, both through it's nature and sign (the 5th might stretch into Virgo).
Thus if the 5th also contained Saturn, there would be a more rigid and formal approach than if it were not there. Saturn in Leo is debilitated, so this would not be a particularly helpful trait, too much concentration on the rules of the game and not enough pure display of the self.
Obviously, aspects between Saturn and other planets will introduce further modifications.
There is some debate within the tradition as to which is most important for the affairs of the house, ruler or tenant. The mainstream view was the ruler, but the 17th Century Astrologer, J B Morin, said tenant. Morin's view seems to be the main view now, but that doesn't mean the issue is still not a real one.
I only use the tradition rulers and ignore the outer planets for this exercise. That's because there is no real theory behind the use of the outer planets as rulers, they've sort of been grafted on. Even those who do use them recognise them as co-rulers, so it's easier to keep to the traditional 7 planets.
Only use the outers if there is a clear message that cannot be deduced from the traditionals.
