In general, a sub-account acts just like its name suggests: a smaller account carved out from a larger one. A separate place to store money. Like...you have most of your savings under the mattress, but a sub-account of quarters buried in the backyard.
There's a more elaborate definition for "sub-account" that comes up in a specific connotation. You can set up a sub-account with your bank, creating a separate place to put money, but with very specific rules about the money's use. Basically, you make an agreement with the bank detailing how the funds should be used, and the bank manages the funds according to those instructions.
