Your answer can be answered in two different ways i think u mean .
When the Computer are made , we have a mother board which consist of all the important software all program to run the computer system or to start it up .
We have C++ which is our by default programming language which knows what to do when certain set of command error free and in proper syntax . and hence the computer perform the program
The computer has its own language which is known as computer language which is of the form 0 and 1 . each letter and each number has its own significance in terms of these digits which in term gives the computer instruction and help him to know that we mean 'a' or 'b' or anything else .
And if you are new to this stuff better concentrate in your classes its a beautiful subject