Some newbie question

Hello

Just trying to play around with PostgreSQL. Some commands and their
result are:

test=# create table ncbi ( id serial, title varchar(50), abstract text,
primary key(id));
NOTICE:  CREATE TABLE will create implicit sequence "ncbi_id_seq" for
serial column "ncbi.id"
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index
"ncbi_pkey" for table "ncbi"
CREATE TABLE
test=# drop table ncbi;
DROP TABLE
test=# create table ncbi ( id serial, title varchar(50), abstract text);

NOTICE:  CREATE TABLE will create implicit sequence "ncbi_id_seq" for
serial column "ncbi.id"
CREATE TABLE

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q1) Does 'implicit index' means that they have given a default name to
the PK which I defined in the first query since I didnt give a name or
does it mean one another index is created apart from the PRIMARY KEY?

Ritesh