A rhotic accent is one where the speaker pronounces a post-vocalic /r/ (ie one occurring after a vowel) that occurs in the written form but is not pronounced by speakers of 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, while in British English accents such as Scots and Somerset are rhotic.