Could smn please explain his joke? I’m stupid I can’t get it :C

probably makes more sense in Korean

If you mean the "will" joke, it's a world play between "will" as someone's last will and testament (a noun refering to a document where someone leaves their money to someone else after their death) and "will" as an auxiliary for moral obligation, as in for example: "you will respect your elders (because that's what you're expected to do)".

When the mc said "you should respect your elders, because there's a will", he deliberately made it sound like he was going to tell the director some big secret about some elder person from the company having included him (the director) in his last will and testament (and who could probably get a bad impression if he saw the director mistreating his "elders", deciding to remove him from the will, as a consequence) when what the mc only had to add was that the "will" was just the common social rule: "If you want to be a decent citizen, you will respect your elders..."