What are Identity Columns?
This is to implement the feature found in the standard. (copied from a draft, date: 2011-12-21):


  4.15.11 Identity columns
  
  The columns of a base table BT can optionally include not more than one identity column. The declared type
  of an identity column is either an exact numeric type with scale 0 (zero), INTEGER for example, or a distinct
  type whose source type is an exact numeric type with scale 0 (zero). An identity column has a start value, an
  increment, a maximum value, a minimum value, and a cycle option. 
  ...
  ...  The definition of an identity column
  may specify GENERATED ALWAYS or GENERATED BY DEFAULT.


It is a property of a column which basically says that the values for the column will be provided by the DBMS and not by the user and in some specific manner and restrictions (increasing, decreasing, having max/min values, cycling if the max/min value is reached).

Sequence generators (usually called just "sequences") are a related SQL standard feature: it's a mechanism that provides such values - and can be used for identity columns.

Note the subtle difference: a SEQUENCE is an object that can be used to provide values for one or more identity columns or even at will.



The various DBMS have so far implemented similar features in different ways and syntax (MySQL: AUTO_INCREMENT, SQL Server: IDENTITY (seed, increment), PostgreSQL: serial using SEQUENCE, Oracle: using triggers, etc) and only recently added sequence generators (SQL Server in version 2012 and Oracle in 12c).

Up to now Postgres has implemented sequence generators (which can be used to provide values for column, either with the special macros serial and bigserial or with nextval() function) but has not yet implemented the syntax for identity columns, as it is in the standard. 

Defining identity columns (and the slight difference from serial columns) and various syntax (eg. GENERATED ALWAYS, NEXT VALUE FOR, etc) from the SQL standard is what this feature is about. Some changes / improvements may need to be done on the implementation of sequences as well, as identity columns will be using sequences.

If you follow the link identitity columns (from the page you saw), you'll find:


  identity columns
  
  From:     Peter Eisentraut 
  To:   pgsql-hackers 
  Subject:  identity columns
  Date:     2016-08-31 04:00:42
  Message-ID:  6adbacbf-73bc-dd1a-2033-63409180fd18@2ndquadrant.com  
  
  
  
  Here is another attempt to implement identity columns.  This is a
  standard-conforming variant of PostgreSQL's serial columns.  It also
  fixes a few usability issues that serial columns have:
  
  
  need to set permissions on sequence in addition to table (*)
  CREATE TABLE / LIKE copies default but refers to same sequence
  cannot add/drop serialness with ALTER TABLE
  dropping default does not drop sequence
  slight weirdnesses because serial is some kind of special macro
  
  
  (*) Not actually implemented yet, because I wanted to make use of the
  NEXT VALUE FOR stuff I had previously posted, but I have more work to
  do there.
  
  ...


Update 2017, September: seems like the feature will be in Postgres 10, which is to be released in a few days/weeks:  What's New In Postgres 10: Identity Columns



Oracle have also implemented identity columns and sequences, in version 12c. The syntax is according to the standard, as far as I checked:
Identity Columns in Oracle Database 12c Release 1 (12.1)


  The 12c database introduces the ability define an identity clause against a table column defined using a numeric type. The syntax is show below.

GENERATED
[ ALWAYS | BY DEFAULT [ ON NULL ] ]
AS IDENTITY [ ( identity_options ) ]