R-dropping also called non-rhoticity or post vocalic r-lessness happens to English-speakers when the "r" sound is not pronounced after a vowel. Then the words "car" and "card" both sound like "cah" and "cahd". That happens for most speakers in England and some speakers in the United States, especially in the Boston and New York City areas and pockets of the American South especially the Greater New Orleans area, Virginia's Tidewater region and among speakers of ethnolects such as African American Vernacular English and Cajun English. British and Northeastern r-droppers usually pronounce "r" when the next word begins with a vowel as in "car is". For them, "spa" and "spar" sound alike, Southern, African American and South African r-droppers don't add additional Rs.
The pronunciation of the R is kept in the Scottish, Irish and most of the U.S. and Canada versions of spoken English. The R-droppers include most of England, Wales, Australia, New Zealand and South Africa.
The issue of how the R is used is called rhoticity by linguists.