Trouble understanding the concepts behind the code

For the assignment, I have to add upon the previous by including a function to detect that the characters inputted are appropriate. They can't be letters or symbols, can only be numbers, one decimal, and only two places from the decimal.

Before I start writing code and make it really obvious I don't know what I'm doing, I'm trying to get the concepts through my head with pseudocode. I can't even get the pseudocode down because I have trouble getting the concepts.

I know in order to check characters, there needs to be conversion going on-- but why? Is it to check values? To store data? I don't understand.

This is where I'm stuck. A classmate told me the code has to check for fractions to make sure the decimal isn't over 100 but beyond that it went into one ear and out the other. I can't write pseudocode if I can't get the basic concepts. Can anyone help me out?