The Wiktionary defines the verb lock up in the second entry as:
to invest in something long term.
Does it mean:
to invest in something that will become apparent later.
to invest in something that will stand for a long term.
There is a technical investment term around the general concept that's called a lock-up period.
The Corporate Finance Institute describes this as:
A lock-up period, also called a locked-up, lock-in or lock-out period, refers to the predetermined time frame in which corporate insiders, investors, and employees are not allowed to sell or redeem their shares after an initial public offering (IPO). It normally happens in instances where a private entity offers its initial public stock issuance.
However, in more general terminology, something that is "locked up" just means that it's either not easily accessible or not accessible at all until a certain period of time has passed.
Other examples of this include certain types of retirement savings that cannot be accessed in any way until somebody reaches a certain age or they die (at which point their estate would get the funds). During this waiting period, the funds are locked in place or locked up.
It's just a figurative term to mean that the money is being kept somewhere and can't go anywhere for a while (either easily or at all). If funds are locked up, they are not generally available for a long period of time.
Often, although not always, such funds will accrue interest or be linked to stock markets or other forms of investment. If the investor is lucky, when the long-term period of waiting is over, they will get out more funds than they put in.