(someone) lost money

You "lose money" when you invest in something, like stocks or businesses, that then become less valuable:

At one point, he had more than 100 million dollars, but he lost it all through bad investments and poor management.

You can also "lose money" by gambling.

a substantial amount of money

The word "substantial" means something between "some" and "a lot". For example, if someone says they lost "a substantial amount of money", that probably means 10% or more of their wealth. Another example:

I earned a substantial amount of money as a teenager through buying and selling comic books at different trade shows and conventions.

Here are some other things that you can have "a substantial amount of":

• spend a substantial amount of time (on something)
• a substantial amount of evidence (that something is true)
• a substantial amount of work (is being done)