Whenever a body moves in a fluid, its interaction with the fluid generates a force on the body. This force is decomposed in a component perpendicular to the direction of movement plus a component parallel to it. The first component is called lift and the second one is called drag. The lift can be positive or negative while the drag is always positive in the sense that it always act against the movement of the body i.e. it always tends to slow down the body. Both these components depends (among other things) on the relative orientation $\alpha$ of the body with the flow. $\alpha$ is called angle of attack. If the body has as shape (like an airfoil or even an A380) such that it generates 1)lift in a controlled way and 2)drag in an amount as small as possible, lift and drag have a quite typical trend versus $\alpha$:
Lift and drag of NACA 0012 airfoil, from NACA-TR-460. Source: https://ntrs.nasa.gov/citations/19930091108
In this picture I highlighted the drag in red and the lift in blue. As said, drag is always positive and lift can be both negative and positive. The lift has a typical linear trend with $\alpha$ while drag has a typical u shape. An important thing to observe here is that for each $\alpha$ there is only one value of lift and only one value of drag. That implies that we can get rid of $\alpha$ and plot lift versus drag: this plot is exactly the polar (highlighted in yellow in the following picture):
Polar of NACA 0012, from Improving Airfoil Drag Prediction by Ramanujam, Özdemir and Hoeijmakers. Source: https://www.researchgate.net/publication/304104830_Improving_Airfoil_Drag_Prediction/download
The answer to this question:
What are these points in this drag curve ("Lilienthal'sches Polardiagramm")?
gives a nice explanation of the meaning of some points laying on the polar, highlighting the usefulness of the polar diagrams.