Logical block addresses.
Lets assume we have an OS and a drive with 100 LBAs and LBAs 1-50 are in use. Now, the LBAs are only used to communicate between the OS and the drive controller -- the drive controller needs to do its own tracking to know where the data is on the actual platter and uses sectors for that (to keep this simple lets assume the drive has 100 sectors and sectors 1-50 are now in use).
If you go and delete a file that uses LBAs 4, 7, 35 and 42, the OS marks on its own mapping table that these LBAs are now available. The drive still thinks these are in use (which is why we need TRIM with SSDs).
Then, if you go and write a file that needs for example 7 LBAs (4, 7, 35, 42 and 51-53), the drive doesn't necessarily write to the corresponding sectors (i.e. 4, 7, 35, 42 and 51-53) because writing to non-sequential sectors is slower (this is very drive dependent). Instead, the drive controller may choose to write to sectors 51-57, which means LBAs 4, 7, 35, 42 and 51-53 are now mapped to sectors 51-57. The drive controller will also know that sectors 4, 7, 35 and 42 are now empty and it can map new LBAs to them (e.g. 57-60).