This is the 334th most frequent Portuguese word.
serem
"Serem" is the infinitive form of "ser" (to be) conjugated in the personal infinitive for "they" or "you (plural)."
Here, 'serem' is the infinitive form of 'ser' conjugated to match the subject 'eles' after the preposition 'de'.
Eles precisam ser felizes antes de serem saudáveis.
They need to be happy before being healthy.
'Serem' complements the verb 'devem' and is conjugated to align with the plural subject 'as janelas'.
As janelas devem serem fechadas ao sair.
The windows should be closed upon leaving.
In this case, 'serem' is used after the verb 'precisam' to describe what things need to be.