What is a page in 8051 microprocessor?
In short, and in this context, the whole 11-bit address area that is accessible with the ACALL instruction is divided to eight areas called pages that are 256 bytes each.

As the ACALL opcode follows only a single byte which defines the 8 least significant bits of the address, there needs to be 8 different ACALL opcodes that define the high 3 bits - the page.