grep of course is a command that searches text (either in files or piped in from another command) for matching strings. It can match either straight text or use regular expressions
for more flexible searches.
By default grep will give you a whole line that contains a match, but in this instance, -v means invert it; show only lines that don't match.
The second part is the string to match. "^" is a regex character that matches the beginning of a line, so the pattern is any "+:" found at the beginning of a line.
Finally, /etc/group is the name of the file to search.
So, this pattern means, "show every line in /etc/group except those that begin with a "+:".
Do a google search for "grep tutorial". You'll find a load of information.