'Tal' is the adjective 'Tall', If it is used with a noun then it immediately follows the noun, eg 'dyn tal', (tall man/ a tall man) if it is used in a sentence then you need 'yn', eg 'mae'r dyn yn dal' (the man is tall)

In the sentence 'yn' links the verb 'mae' with the adjective. literally the meaning is 'The man is being tall'

This example above is to indicate how 'yn' softens the adjective 'tal'.

Dal, the verb has a number of meanings including the one in your example.

Tal is the adjective for tall, this is mutated after 'yn' to dal.