With a block given, forms the substrings ("lines")
that are the result of splitting +self+
at each occurrence of the given line separator +line_sep+;
passes each line to the block;
returns +self+:

  s = <<~EOT
  This is the first line.
  This is line two.

  This is line four.
  This is line five.
  EOT

  s.each_line {|line| p line }

Output:

  "This is the first line.\n"
  "This is line two.\n"
  "\n"
  "This is line four.\n"
  "This is line five.\n"

With a different +line_sep+:

  s.each_line(' is ') {|line| p line }

Output:

  "This is "
  "the first line.\nThis is "
  "line two.\n\nThis is "
  "line four.\nThis is "
  "line five.\n"

With +chomp+ as +true+, removes the trailing +line_sep+ from each line:

  s.each_line(chomp: true) {|line| p line }

Output:

  "This is the first line."
  "This is line two."
  ""
  "This is line four."
  "This is line five."

With an empty string as +line_sep+,
forms and passes "paragraphs" by splitting at each occurrence
of two or more newlines:

  s.each_line('') {|line| p line }

Output:

  "This is the first line.\nThis is line two.\n\n"
  "This is line four.\nThis is line five.\n"

With no block given, returns an enumerator.