A sequence of instructions in a program that the processor repeats, either until some condition is met, or indefinitely (an infinite loop).
In an structured language (e.g. C, Pascal, BASIC, or Fortran), a loop is usually achieved with for loop, while loop or repeat loop constructs.
In other languages these constructs may be synthesised with a jump (assembly language) or a GOTO (early Fortran or BASIC).
To "loop through" a list means to process each element in turn. Hackers might use this for everyday iterative actions, e.g. "I've got to loop through my paper mail."