Hi Hoan,
You mean this part?
ISP1(config)#route-map PREPEND_EU permit 10
ISP1(config-route-map)#match community 1
ISP1(config-route-map)#set as prepend 1 1 1 1
ISP1(config-route-map)#exit
ISP1(config)#route-map PREPEND_EU permit 20
A route-map starts at the top and works it way to the bottom. First, it checks for everything in statement 10 and if it matches community 1, it prepends the AS path.
Then it processes the permit 20 statement which is empty. An empty statement means “match everything” so the end result will be that we advertise all other routes without changing the AS path.
If you don’t add this empty statement then everything else will be denied by default, just like an access-list. What happens is that only the networks that are matched in statement 10 get advertised and nothing else.
Hope this helps!
Rene