In order to prevent loads of perfectly innocent people being locked up, the law gives all suspected offenders the right to bail unless there are specific reasons for bail not to be granted. (e.g. bail can be refused when there would appear to be a likelihood that the suspect will flee the country, when it's thought possible that he might carry out other offences or when he might seek to interfere with witnesses). So the vast majority of suspects in criminal investigations are given bail (or 'released under investigation' as below).
Some time ago though, there were concerns that many suspects were being given loads of restrictive bail conditions and that they were then constantly being re-bailed, meaning that their lives could be disrupting for many months or even years before a decision as to whether to proceed with any criminal charges was made. (Innocent people were losing their livelihoods and, if their businesses went bust because of the bail conditions, their employees would suffer too).
So the police are now only allowed to bail suspects for a fairly short period of time. As an alternative, they can now 'release a suspect under investigation', meaning that they can be required to attend a police station at an unspecified date but they can't be made the subject of any bail conditions.