When you post a transaction using movement type 101 (as an example), there are so many events associated with this posting. Event means it will be hitting so many types of accounts. These are called trandaction events.
E.g. BSX, GBB, WRX etc.
This is something which modifies the accounts by addiding BSA, ZZZ, ZDF etc. Before the account modifier was used, let us say, posting was done to account 400000. If you want the same posting to go to another account, then you can add an account modifier ( A 3 LETTER WORD ). Then it will post to another GL account.