I earn less than 6k a year but ive paid over 1k in tax. Why?

so ive researched a few sites and it says that if you earn less that approx �6,450. you dont pay tax. yet ive been paying 20% tax because my tax code is BR. why is it BR. i dont have another job, i dont pay into a pension. Why am i paying tax when i earn less than the taxable income limit. if someone can help me with what i should do all help is appreciated. if you could also tell me what the tax code BT means and what qualifies to be in that tax margain. thanks