It will obtain a shared lock on your tables when you run the pg_dump. Any transactions completed after you run the dump will not be included. So when the dump is finished, if there are current transactions in process that haven't been committed, they won't be included in the dump.
There is another param with which it can be run that says this
--lock-wait-timeout=timeout Do not wait forever to acquire shared table locks at the beginning of the dump. Instead fail if unable to lock a table within the specified timeout. The timeout may be specified in any of the formats accepted by SET statement_timeout. (Allowed values vary depending on the server version you are dumping from, but an integer number of milliseconds is accepted by all versions since 7.3. This option is ignored when dumping from a pre-7.3 server.)**