If a person is arrested, it means that the police have lawfully taken that person into police custody.
If you are arrested, you will be required to accompany the police to either a police station or a watch-house. There, you will be processed and then either taken to court at a later stage (most usually the following morning, if possible) or released on bail. If a person is released on bail, it means that they are released back into the community while waiting for the next court date. However you do not need to accompany the police if you have not actually been arrested or formally detained for questioning about an indictable offence.
A person can be arrested if a warrant has been issued for their arrest. This often happens if a person does not attend court when required and is therefore in breach of their bail undertaking. There are also a number of situations where a person can be arrested without a warrant, such as where the police reasonably suspect the person is committing an offence and they believe that arresting the person will cause them to stop committing the offence.
The arrest of a person means that their liberty has been deprived. For this reason, the police should only arrest a person in accordance with strict policies and procedures.