It's just a wiffleball. Probably a soccer mom thing but for young children's T-ball.
My guess is that they glued it to a magnet ( the disc ), and stuck it on their car for a sense of identity but without causing any irreparable damage to the car. These little things that give identity also have a side-convenience of making their car easier to spot in a parking lot full of cars that all look the same.
Here's a similar one (description says 12", but the pictures make it look closer to the car picture) https://www.amazon.com/Crown-Sporting-Goods-12-inch-Softballs/dp/B07CG8RT1J
Edit: Or a pickleball seems to have holes that are closer to the same diameter as in the one in the car picture, but that seems less likely to me just because pickleball is less common in my area. https://www.amazon.com/gp/aw/d/B01MDTU83P
Product name: GoSports 2.85" Weighted Training Baseballs - Hitting & Pitching Training for All Skill Levels - Improve Power and Mechanics
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Previous price: $27.99
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The item was in stock as of 23/05/2021 18:10:49 (UTC)
While /u/thepowerisyours sockballs are an intriguing free option, I'd recommend getting these: SKLZ Impact Softballs.
They're light, don't fly very far (80-90ft), and unlike regular wiffle balls they're made with tough plastic so they last much longer. They also deform significantly when you hit them, so you know immediately when you haven't squared one up.