when we write...?

when we can write" tú" and "vosotros" this two world have same meaning

I am not sure what you are asking because your question is unclear. I shall try to respond to the question I think you are asking. Tú and vosotros do not have the same meaning. Vosotros is "tú" in the plural form. Furthermore, it is mainly used in Spain. In Latin America, the formal plural form of "Ustedes" is used for both formal and informal uses of the plural forms of "tú" and "usted." Think of it this way. Tú means "you" and vosotros means "you all." In English we often just say "you" when we mean "you all." In Spanish you cannout use "tú" to say "you all."

so vosotros means tú but plural?

Yes that is correct. It is informal plural you.