Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line is read and returned. In list context, reads until end-of-file is reached and returns a list of lines (however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR). This is the internal function implementing the <EXPR> operator, but you can use it directly. The <EXPR> operator is discussed in more detail in "I/O Operators" in perlop.
$line = <STDIN>;
$line = readline(*STDIN); # same thing