An ELT Glossary : Rhotic Accents
Rhotic accents are those where a post-vocalic "r" (ie one occurring after a vowel in the written form) is pronounced. This does not happen in non-rhotic accents.
For example, the words where, hard and butter would be pronounced as /weə/, /hɑːd/ and /bʌtə/ in a non-rhotic accent but as /weər/, /hɑːrd/ and /bʌtər/ in a rhotic accent.
Many N. American accents are rhotic, as are some British accents - eg Scots and Somerset - though most British accents and other varieties such as Australian English are non-rhotic..