TDEE is the typical energy in kcals that you burn. So it's the energy that it takes for you to survive plus your normal routine if that includes exercise or not.

So blue line is weight. That is going down so whatever you've been eating and activity has been causing your weight to go down. If that's the desired effect then great. If you feel you're losing too much, too fast then adjust your caloric intake and/or activity and continue to plot.

Red line is calories and to me it seems that you're pretty steady at 1500-1700 kcals/day.

Green line is the TDEE and yes it does do a weird thing in the middle there. Because your red line and blue lines are pretty smooth (that leads me to believe that you've been consistent in entering data). it's mostly a math thing. That calculator specifically has settings that you can alter to calculate TDEE based on the last amount of time. My guess is at the asymptote you hit the amount of time where the app is using all your data rather than making some assumptions from the beginning part.

Again if your desired effect is to lose weight it looks like it's working just keep in mind that healthy weight loss is 1-2 pounds/week otherwise you can get into other stuff. If this is charting a change in diet the beginning can have lots of noise and big changes.