There are a couple of considerations
1) The front end generally contains queries, reports, and forms
2)The back end generally contains tables (i.e. data)
3) If someone is using the front end, they (in general) are accessing data in the back end. In other words, if you are using the front end, you are also using the back end. This is not 100% the case, but in normal database usage, it is.
Updating information in the backend while users are accessing on the front end may or may not be a problem. For example, if a user is accessing Form A in the front end that is based on table A in the back end, you will most likely have problems if you try to change a datatype in table A and save it. If however a user is using form B based on table B and you update table A (which no one is using), you should have no problem.
Let me know if you have questions on this or need clarification.
Jay