Economics is the study of the efficient allocation of resources. There are a very large number of variables with complex interactions between them. Econometricians design models to represent an economy - or some aspect of it - and then use statistical methods to estimate relationships between the variables.
Usually the expression is employed in the context of the relationship between a dependent variable and another variable. The latter may or may not be independent: often it is time but that is not necessary. In some cases there is some indication that that there is a linear relationship between the two variables and that relationship is referred to as a trend.Note that a trend is not the same as causation. There may appear to be a strong linear trend between two variables but the variables may not be directly related at all: they may both be related to a third variable. Also, the absence of linear trends does not imply that the variables are unrelated: there may be non-linear relationships.
who cares about that poooooop
Some people will give the answer "correlation". But that is not correct for the following reason: Consider the coordinates of a circle. There is obviously a very strong relationship between the x coordinate and the y coordinate. The correlation between the two is not just small, but 0. The correlation between two variables is a measure of the linear relationship between them. But there can be non-linear relationships which will not necessarily be reflected by any correlation.
It means that the variables are related in some way; one affects the other.
A line graph can tell you how changes in one variable are related to changes in the other. A line graph cannot show causality. A line graph can show non-linear relationships which some other analytical techniques may not identify. In particular, they are good for identifying relationships between the variables that change over the domain. A line graph can also help identify points where the nature of the relationship changes - eg tension and breaking point, or temperature and phase. The spread of observations about the "line of best fit" gives a measure of how closely the variables are related and how much of the measurement is systemic or random error.
by their increase or decrease
sea turtles have friendly relationships with other sea turtle species, and frock
Decision variables are the variables within a model that one can control. They are not random variables. For example, a decision variable might be: whether to vaccinate a population (TRUE or FALSE); the amount of budget to spend (a continuous variable between some minimum and maximum); or how many cars to have in a car pool (a discrete variable between some minimum and maximum).