An old, dead form of the verb *be*: it means "(you) were", used only with the old singular *thou*. So "thou wert" = "you were". You will meet it in the King James Bible, in Shakespeare, in old hymns and old poems — never in real speech, and never in anything written today unless the writer is copying an old style on purpose. A little history helps here: English used to have two words for "you". *Thou* was for one person you were close to or above; *you* was for several people, or for one person you had to be polite to. Each had its own verb ending, and *wert* was the past of *be* that went with *thou*. English later dropped *thou* and used *you* for everybody, and when *thou* went, *wert* went with it. Two small notes. Older writers also used *wast* for the same job ("thou wast"), and the two sat side by side for a long time; roughly, *wert* was more common where the meaning was "if you were" or "I wish you were", but writers were not consistent, so do not trust a neat rule here. And *wert* is also the modern German word for "value" (as in *Wert*), which is a different word entirely — a spelling accident, not a family link. For your own English, this is a reading word, not a writing word: recognise it, then move on.