"Sinun pitäisi" means "you should" so in other words, it is a conditional. You can tell that from the "-isi" part in the verb. In English we have would/should/could before the verb, but in Finnish the conditional marker is added in the verb itself. Therefore it is slightly different from "sinun pitää", which would mean "you must" or "you have to". The other one should be "sinulla pitäisi" so no "ä" in the first word. It can be used for example in a sentence like "Sinulla pitäisi olla tarpeeksi rahaa" = "You should have enough money"

Hi Dear Sarah! So happy to see you online! ;)