when a table or record needs to be queried or updated, while data is in-flight, the table (or record, or page) is locked so that data integrity (ACID) can be assured.
SELECT * FROM sys.dm_tran_locks
to examine the locks A lock might create a block on other process (the second can’t finish until the first is done and the lock removed. this might also involve a deadlock based on two processes – one will be terminated and the other will eventually complete).
Create in testing by starting a transaction without a commit tran or rollback tran.