An algorithm is a set of instructions for solving a problem or completing a task.
With mathematics, algorithm often refers to the procedures for solving a recurring problem. The steps for working out long division for example; solving for missing variables per the Pythagorean theorem is another example.
All of the science and math that underpins the workings of computer programs, web sites, even electronic elements of modern appliances, automobiles, digital thermometers, etc. is governed by highly complex algorithms. You use "a+b=c" all the time, even if you are not aware of it, the programmers have made it easy for all of us.