Hi Stefano,

> I can't understand what exaclty þats is. Maybe a contracted form

from þat es = þat er ?

That's exactly right.

Nú er þats Boðnar bára, berg-Saxa, tér vaxa, gørvi í höll ok hlýði
hljóð fley jöfurs þjóðir.

"Now it is that [time] when" (i.e. "Now" or "Now is when") "the wave
of Boðn" (=[my] poetry) "does grow. Let the prince's men be silent in
the hall and listen to the ferry of rock/cliff-Saxons" (=ferry of
dwarves = poetry).

Boðnar bára "Boðn's wave". Boðn was one of the vessels that held the
mead of poetry; its wave is the liquid it contains: the poetry itself.

gørvi hljóð "let them make silence" (i.e. be silent).

hlýði fley berg-Saxa jöfurs þjóðir "let the prince's men listen to the
ferry of rock/cliff-Saxons" (=ship of dwarves = poetry). As `hlýða'
normally takes dative, Faulkes suggests the final vowel of `fleyi'
might have been elided before `jöfurs'. Konráð Gíslason suggested
amending `hlýði' to `heyri' "let...hear", which takes accusative.

In his Skjaldedigtning, Finnur Jónsson translated it into Danish as:
Nu vokser mit dikt; fyrstens mænd give lyd i hallen og lytte til digtet.

LN