Cash is the main currency of Top Eleven. A manager needs to spend it when doing any of the following:

Buying a player on an auction or from the Scout List. Unlike tokens, cash is spent only after the manager has successfully won the auction.
Building stadium facilities.
Paying wages for players.
Paying win bonuses to players.
Sacking a player.

Cash can be earned in a variety of ways:

Signing a sponsorship deal.
Selling a player on the Transfer list.
Earning prizes from competitions.
Ticket sales.
Selling merchandise.
Getting a Bank Loan from in the Club Shop menu.
Completing various offers and surveys from affiliated partners.
A clear overview of how much cash you earn and spend can be found in the Finances menu.