Cash is one of the in-game currencies, and is gained by various means. These include missions, defeating NPC's, and selling items to vendors. It has many uses, but is primarily used to repair damaged equipment or purchase items from vendors.
For free accounts there's a limit of $1500, for Premium accounts it is $2000 and for Legendary accounts there is no limit for Cash. It should be noted that if the player exceeds this limit, the overflowing cash is locked in Escrow and can only be accessed by upgrading the account.
Cash is the currency used to buy and sell items on the Broker.