"To invest the rest" means using the remaining amount of money or resources to put it into something that has the potential to grow or increase in value over time. Full definition
As for investing the rest of his liquid cash, he's not sure. (moneysense.ca)
Use the excess capital to reach financial freedom by paying off your debts and by investing the rest in income generating assets. (quityourdayjob101.com)
Then invest the rest amount in any other fixed return plan to get maximum benefit. (mydailylifetips.com)