Translation:You do not need to listen to the grandmas.
The pronunciation of vowels is long for open syllables (like aa) and short for closed syllables. An open syllable ends in a vowel and a closed syllable ends in a consonant. The singular "oma" ends in an "a" and therefore it is pronounced as the longer "aa" sound. The plural ends in an "s" but to show that it is still pronounced as the longer aa, an apostrophe is added.
the oma's is to make OMA plural, but keeping the last vowel "A" long. For more reference, check here:
We only use the possessive 's' with names, so "Jans hond" would be fine. However, we don't use the apostrophe for possession... except when the name ends on a single long vowel (like Anna), and when the name ends with an -s or -s sound (like Jos, Sjostakovitsj, Jacques, Waltz), but then you only use the apostrophe.
So, the names above look like this:
- Jans hond
- Anna's hond
- Jos' hond
- Sjostakovitsj' hond
- Jacques' hond
- Waltz' hond
Edit: Besides names, you can also use it with some family members, but only if you use a possessive (like "mijn") before the family member:
- Mijn moeders auto
- Haar vaders baan
- Jouw broers vriendin