1- That makes sense.
what’s the meaning of this sentence?
It means that "that looks OK" or "that seems to fit". It depends on the context.
Try to understand it by the context.
"The eingenvector of an operator cannot be the null vector."
"Because if that were the case, any scalar would be an eingenvalue."
"Hmm, that makes sense."
"I think that the number one should be considered a prime as well."
"Oh really, why do you think so?"
"It is only a multiple of itself and of one."
"But then there would be no other primes besides one, think about it."
(after a few secs)
"Yeah, that makes sense. I think you are right."