what's kind of reminder is???

It's the remainder; ie, the bit left over after the integer quotient given in A!

Back to basics:
If you try to divide fifteen apples into four equal piles, you will get three in each pile and three left over;
Mathematically, 15/4 = 3 plus a remainder of 3

We're just dealing with 8-bit numbers here; whether you choose to represent those numbers in binary, octal, decimal, hexadecimal or any other notation is irrelevant to the DIV operation:

Binary: 00001111/00000100 = 00000011 + remainder 00000011
Octal: 017/004 = 003 + remainder 003
Decimal: 15/4 = 3 + remainder 3
Hex: 0x0F/0x04 = 0x03 + remainder 0x03
Roman: XV/IV = III + remainder III

..."For find real decimal reminder..."

When you say "real" do you mean "floating-point?"
The 8051 has no floating-point instructions; you would have to write (or otherwise obtain) floating-point routines.