The code example on the first slide covers the use of `sizeof` operator. The `sizeof` operator returns number of bytes used (needed) to allocate a certain type, which is passed in as the operator's argument.
Bro are you kidding me? you have 40+ codes but you say you don't know what a byte is?
About %ld, we call it format / conversion specifiers. It's used basically to describe which data type to be used when reading or printing data.