You have two variables:
X₁: The number of people in a household.
X₂: The amount of the grocery bill of the household.
To determine the type of relationshipvmay exist between these values you have to use a little logic.
Each person uses a determined number of groceries.
If a person lives alone, then this person will buy a determined number of groceries, x. If a second person moves into the house, the amount of groceries needed will increase, because now they have to buy groceries for two people.
If a third person moves in, then the groceries needed for the household will increase.
And so on, so you can say that the greater the number of people living in a house, the greater will be the groceries bought and therefore, the grocery bill will also increase.
If both variables have a direct relationship, i.e. they increase jointly, then you can say that they are positively correlated.
So, the relationship between the number of people in a household and their grocery bill is positive.