'BACS' and 'CHAPS' are payment terms. 'BACS' is the standard payment and the one that will be used by most Umbrella companies. When the Umbrella company pays you, a BACS payment will take three working days to reach your bank account.
The CHAPS payment will arrive into your bank account the same day that payment is made. Banks charge for CHAPS payments, so it is likely that your umbrella company will pass this charge onto you.