is
The word 'ist' is the third person singular form of the verb 'sein' in German, which means 'to be'. It is used to express the present tense of 'to be' for a singular noun or pronoun. For example, 'Er ist ein Lehrer' means 'He is a teacher'.
The weather is very good today.
This sentence is a basic method to express the status of the weather today in German language. 'Das Wetter' means 'the weather', 'ist' means 'is', 'sehr' means 'very', and 'gut' means 'good'. The word 'heute' stands for 'today'.
Berlin is the capital of Germany.
This sentence states a fact about a certain city belonging to a certain country. 'Berlin' is a city name, 'ist' means 'is', 'die Hauptstadt' means 'the capital', 'von' stands for 'of', and 'Deutschland' is the name of the country 'Germany'.