INT 13H is shorthand for BIOS interrupt call 13hex, the 20th interrupt vector in an x86-based computer system. The BIOS typically sets up a real mode interrupt handler at this vector that provides sector-based hard disk and floppy disk read and write services using cylinder-head-sector (CHS) addressing.
INT is an x86 instruction that triggers a software interrupt, and 13hex vector passed to the instruction (interrupts start at zero, and are labeled with hexadecimal values).
Read more about INT 13H: Overview