bitlocker ? Am scared

What does bitlocker do ?
Does it encrypt access to hard drive or to all files on hard drive ? Because am not sure how software will handle it.
If i bitlock one of my 5 hard drive and format my windows and reinstall windows, will i be able to use bitlocked hard drive ? How, will i have to enter the password or what ? Each time i access it ?

Thanks

Oh never mind, its encrypting each file, but i wonder, does it encrypt files by adding information to file or changing its attributes ?
Caz it takes forever and i dont think it just clocks HDD access and i think its gonna take just as long to decrypt