What does it mean to "rack in some serious cash"? Does it mean to make money quickly and making loads of cash?
Yes. It means making a lot of money fast.
Rake in means
Fig. to take in a lot of something, usually money.
"Our candidate will rake votes in by the thousand."
"They were raking in money by the bushel."
Rack up means
Fig. to accumulate something; to collect or acquire something.
"They all racked a lot of profits up."
"We racked up twenty points in the game last Saturday."
What you heard is probably a mix of these two.
Also, Serious means
Informal Of considerable size or scope; substantial
"a serious amount of money."
Definitions from The Free Dictionary
To rake in means:
to earn or get a large amount of money: 'He rakes in over $200,000 a year.'
"Rake in some serious cash" - Making a lot of money quickly and steadily.
She's really raking it in (= making a lot of money).
To rack something up means:
to gradually get more points, profits, etc.: 'He has racked up 450 points in three months.'
[Cambridge Online Dictionary]