It's English it's called an underdot. It is a diacritical mark much as acute accents and umlauts are. In Unicode it is referred to as "dot below".
In Vietnamese, it's used to represent a particular tone, as Vietnamese is a tonal language where tone affects meaning and so it is good to reflect it in spelling.
Note that in
ị the underdot is considered a diacritical mark, but the dot above is not; it is considered part of the
How to type it depends on your keyboard. One type used in Vietnamese would type it by pressing the key that is in the position
9 is on English-language keyboards, followed by the letter wanted for
ạ ẹ ị ọ ụ ỵ (a "dead key" method to diacritics, as on physical typewriters the key wouldn't move the paper). It's combined with other dead keys for
ặ ậ ệ ộ ợ ự.
In Unicode, it can be encoded either by "precomposed characters", or by following the vowel with "combinging dot below".