Kinda the same with most photography equipment. An 1.4 certainly isn't 3-10x better than a 1.8, but we sure want it
MTF50 with line pairs per picture height.
MTF50 means you have white and black lines and meassure when contrast between the two is 50% or higher. So a result of 1500 means you can put 1500 alternating black and white lines going from top to bottom of the image sensor and the contrast between the two would be 50% (so plenty to distinguish the lines). It's line
pairs so in total it would mean 3000 lines.
The astigmatism is how different the lens performs with lines that go outward from the center (tangential) or circle around it (sagittal). This can affect bokeh (the Petzval bokeh is caused by astigmatism from the tangential and sagittal plane being focused differently hence why the balls are "squeezed" instead of circular) and smear the edges.
Edit: Don't worry if you don't quite understand that, I don't really either. I tried to read Rudolf Kingslake's "Fundamental's of Optics" and didn't understand a thing. The book also hilariously started showing a double gauss design (what most 50mm lenses use, perhaps the most simple design still in use) and stated that it was much more complicated than what the book would cover...
It really helps with visual aids to see how a lens works and how aberrations affect the image.