Cfrgtkky

When I run cat /etc/group on Ubuntu 18.04, I get this output:

root:x:0:

daemon:x:1:

bin:x:2:

sys:x:3:

adm:x:4:syslog,hani

tty:x:5:

disk:x:6:

lp:x:7:

mail:x:8:

news:x:9:

uucp:x:10:

man:x:12:

proxy:x:13:

kmem:x:15:

dialout:x:20:

fax:x:21:

voice:x:22:

cdrom:x:24:hani

floppy:x:25:

tape:x:26:

sudo:x:27:hani

audio:x:29:pulse

dip:x:30:hani

www-data:x:33:

backup:x:34:

operator:x:37:

list:x:38:

irc:x:39:

src:x:40:

gnats:x:41:

shadow:x:42:

utmp:x:43:

video:x:44:

sasl:x:45:

plugdev:x:46:hani

staff:x:50:

games:x:60:

users:x:100:

nogroup:x:65534:

systemd-journal:x:101:

systemd-network:x:102:

systemd-resolve:x:103:

input:x:104:

crontab:x:105:

syslog:x:106:

messagebus:x:107:

netdev:x:108:

mlocate:x:109:

ssl-cert:x:110:

uuidd:x:111:

avahi-autoipd:x:112:

bluetooth:x:113:

rtkit:x:114:

ssh:x:115:

lpadmin:x:116:hani

whoopsie:x:117:

scanner:x:118:saned

saned:x:119:

pulse:x:120:

pulse-access:x:121:

avahi:x:122:

colord:x:123:

geoclue:x:124:

gdm:x:125:

hani:x:1000:

sambashare:x:126:hani

nvidia-persistenced:x:127:

rdma:x:128:

docker:x:999:

I know that 'root', 'daemon', etc. .. are groups or user names, but what does the rest of that mean (:x:0:, :x:125:, :x:126:, ...)?

Quoting the respective manual page man group:

NAME

       group - user group file



DESCRIPTION

       The  /etc/group file is a text file that defines the groups on the sys‐

       tem.  There is one entry per line, with the following format:



              group_name:password:GID:user_list



       The fields are as follows:



       group_name  the name of the group.



       password    the (encrypted) group password.  If this field is empty, no

                   password is needed.



       GID         the numeric group ID.



       user_list   a  list  of  the  usernames that are members of this group,

                   separated by commas.