An algorithm is a sequence of instructions on how to perform a specific task. The task can be as simple as multiplying two numbers or as complex as building recommendation systems using neural networks.
An algorithm is simply a list of steps to be performed. For example, an algorithm for heating food would be:
Take the food out of the fridge.
Put the food into the microwave.
Set the timer and start the microwave.
Wait until the timer runs out.
Take out the food from the microwave.
Any existing algorithm can be defined within a certain amount of space and time in a well-defined formal language.
In almost every case, there are multiple algorithms that solve a particular problem; the challenge lies in determining the most efficient algorithm for the program to run as fast as possible with minimal use of system resources.
A common practice is to further optimize an already existing algorithm to be released as a future update instead of trying to figure out an efficient algorithm from scratch.