It is easy to confuse words that are written similar and sound similar. Seit and seid are two German words with different meanings.
It should not be to difficult to understand the difference.
Let me explain:
The word "seit" could be translated to "since", it is used in reference to a certain time.
- Seit gestern bin ich krank = I am ill since yesterday.
- Seit wann bist Du so nett? = Since when are you so nice?
"Seid" is the second plural conjugated verb form of the verb "sein", which translates into "to be".
It is also used in verb conjugations in past tenses where the "Hilfsverb" (auxiliary verb) is "to be".
- Ihr seid da = You (plural) are here
- Danke meine Freunde, dass Ihr geblieben seid = Thank you my friends, that you were staying (or that you stayed)