"What is the temperature today?"
Translation:Beth yw'r tymheredd heddiw?
It's a mistake by the contributor. 'Beth ydy' or "Beth yw' are the correct and standard forms of this.
In spoken Welsh you can get 'Be' 'dy' dropping both the 'th' of 'Beth' and the initial vowel of 'ydy'. So this sentence if it was correctly written in a North Wales dialect would be:-
Be' 'dy'r tymheredd heddiw?
It's not uncommon for words that end with "dd" and "th" to have them dropped in colloquial speech e.g "Beth" becomes "Be" and "Bwrdd" (Table) becomes "Bwr".