"Here is a Welsh magazine."
Translation:Dyma gylchgrawn Cymraeg.
dyma/dyna/dacw all cause a soft mutation of the following word. I have just added a note about it to the section on 'Travelling', which I think is where dyma first appears.
According to this (you will need to register for free) - 'there is...'. and a couple of related meanings. Typically something a little way away that you can point at, but different people use it for slightly different things.
There are a lot of dialect variations and usages to do with this type of word.