Hard money has two independent meanings. Both meanings stand for high value.
Economic "Hard Money"
In economic terms, hard money is usually gold and sometimes silver. Hard money policy backs up paper fiat currency such as dollar bills with the right to exchange the paper for the precious metal. In other words, hard money can be exchanged for a set amount of gold or silver. This was popular among conservatives in the late 1800s, and some conservatives, such as Ron Paul, advocate today a return to "hard money" (a specie standard based on gold).
Political "Hard Money"
In campaign finance, hard money means contributions directly to a politician-candidate's campaign for him to spend however he thinks best. This is the most direct type of campaign support that can be given to a candidate. Accordingly, it is the most highly regulated form of campaign expenditure.