A dot above the headstroke in Devanāgarī script represents nasalization or a form of ‘n’ or ‘m’ sound.
अनुनासिक is the nasalization of vowels, which is saying a vowel sound while pushing the air up into your nose. It can be represented by either the symbol ँ which is called चंद्रबिन्दु ('moon-dot'), or the symbol ं which is called बिन्दु ('dot'). If a vowel mātrā written above the line crowds the space then the candrabindu is reduced to a bindu. For instance: हाँ yes, नहीं no, हूँ am, हैं are (NB. है is) Traditionally if there was no other mātrā above the line people used to use candrabindu, but now the use of bindu alone is growing.
Nasal consonants, called अनुस्वार in Hindi, can be represented by a dot above the head stroke of characters, the dots are short hand for full forms.
This symbol represent the English vowel sound ă found in words like dollar and names like John ( जॉन ) and it is only used in formal Hindi, i.e.ॉ as in words like डॉलर dollar. However, many Hindi speakers normally ignore this symbol and would say डालर which it the normal Hindi pronunciation for this word.