An r-squared of 0 would mean that the model does not explain the relationship between the two variables at all. In other words, the relationship between the two variables is completely random, at least according to the regression model.
Alternatively, an r-squared of 0.5 would mean that the regression model explains 50% of the relationship between the two variables.
In general, an r-squared value of less than 0.3 is considered weak and a value above 0.7 is strong.
It's important to consider how well r-squared corresponds to your expectations since there are other factors to consider, such as the nature of the variables in the model and the units of measure.