That code will make it happen for every key except a Carriage Return (Asc Char 13 -- i.e. The Enter key)
Basically asc("t") returns the ascii value of the letter "t". Given any single-character string (string of length one) asc(string) will return the ascii value of that character. Handily enough, that is what you are given when OnKeyPress is called. So you are comparing the given value to what you want it to be. That is, you are checking to see if Keyascii = asc("t"). This equates to checking if KeyAscii = 116 (116 is the ascii value of "t"). So if you want to check to see if they are pressing the spacebar, you could do one of two things:
If KeyAscii = asc(" ") then
or
If KeyAscii = 32 then
I would go with the second one. The reason that the code uses asc("t") is because it makes it easier for the programmer to understand what is being checked for (i.e. if the "T" key is being pressed). Most programmers don't have the ASCII chart memorized, but many do know the ascii value for common characters like Space, Carriage Return, and Double Quote (34). Some also know A is 65, but another level of indirection is sometimes just easier than counting from A to whatever letter you're using.
I hope that helps.