Multiple entry visa means that you have the permission to enter the country/ies where your visa is applicable SEVERAL TIMES within its validity timeframe.
Just take for example the UK standard visitor visa or commonly known in layman’s term as tourist visa. It is valid for 6 months and has a MULTIPLE ENTRY provision. It means that you may enter the UK for as much as you want as long as your entries are within the 6-month visa validity.
You have to bear in mind though that a VISA doesn’t guarantee you quick entry at the border. It still depends on the Immigration Officers to assess if you will be given entry or not.