A cash account is a basic investing account.
It simply is a money market account a broker opens for you that you use to trade stocks with. If you just plan on buying a few stocks here and there this account is all you need. If you plan on being more of an active trader than you should consider a margin account instead.
Cash accounts have many rules in regards to moving in and out of stocks quickly, so if you plan on doing that read the section on margin accounts.