Can’t figure out how to remove this white strip!

I'm completely new to WordPress but I am stuck maintaining the website at my workplace. There's a team that does the general maintenance but I'm stuck with the design and the visuals. Explain how to fix this but please do it like I'm a toddler.

https://preview.redd.it/ygknj5kc86ld1.png?width=1919&format=png&auto=webp&s=d43ee5d4713f46f0d512eccb24b0a20858937460