My last name is Mantta with the dots over the a’s. What is it’s meaning? Thanks

The name Mänttä is also an old place name, which is nowadays a town in Finland. The place name Mänttä comes from an old house which Tuomas Niilonpoika Mäntsä (1570–1618) founded in Keuruskoski in the wilderness of Sääksmäki:
https://en.wikipedia.org/wiki/M%C3%A4ntt%C3%A4

I found two different theories about the name's meaning. One theory is that the word is derived from old Swedish word "mäntare", which means person who process leather or does tanning. So it is possible that the first person with that name was a professional tanner. Other theory is that it might derive from German name Menze.

Tuomas Niilonpoika Mäntsä (Mänttä)'s family has their own society, which might interest you also:
https://mantsa.yhdistysavain.fi/

References:
Mikkonen-Paikkala: "Sukunimet" (Otava, 2000)
https://muinainensuomi.foorumi.eu/viewtopic.php?f=24&t=291