/a:/ is a long, back, open vowel. This sound should mean no problem to you as it is almost identical to the way we pronounce the letter ‘a’ in Spanish when we say ‘avión’.
The defining characteristic of a back vowel is that the tongue is positioned as far back as possible in the mouth without creating a constriction that would be classified as a consonant. Back vowels are sometimes also called dark vowels because they are perceived as sounding darker than the front vowels.
The defining characteristic of an open vowel is that the tongue is positioned as far as possible from the roof of the mouth. Open vowels are sometimes also called low vowels in reference to the low position of the tongue.
Examples of /a:/ are: