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It means that the visa holder can make repeat trips to the UK for up to 180 days per visit during the two year validity period.
What the holder cannot do however, is to effectively live in the UK for long periods of time through frequent or successive visits. This is set out in V4.2(b) of the Immigration Rules https://www.gov.uk/guidance/immigration-rules/immigration-rules-appendix-v-visitor As @jcaron mentions in a comment, this is not as explicitly defined as the 90/180 rule applicable in the Schengen Area for example, but the rule of thumb is the same: you should stay less than half of your time in the UK, and/or you should be out of the UK at least as long as you stayed.
Note that some points used at the time of the visa application to assess whether someone is a ‘genuine visitor’ or is making the UK their main home or place of work or study may also apply when a visit visa holder arrives at the UK border (see “Frequent or successive visits: how to assess if an applicant is making the UK their main home or place of work or study” in the Home Office Visit guidance https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1112745/Visit_caseworker_guidance_12.pdf#page21)