[halvar]

for x in $(seq 1 148) do

means do the following command with the numbers from 1 to 148 (148 times)

This is because

seq 1 148

returns the numbers from 1 to 148. You can execute "seq 1 5" in Terminal to get the idea

$x is a placeholder for the current value.

curl -o $x.jpg http://online.pubhtml5.com/vfof/guzu/files/large/$x.jpg;

is one command which is terminated by ";"
The "-o $x.jpg" is an option meaning save this as 1.jpg, 2.jpg and so on.

A more simple example printing the numbers 3 to 5:

for foo in $(seq 3 5); do echo $foo; done;

Sometimes you need leading zeros:

for foo in $(seq -w 3 10); do echo $foo; done;

You can provide a list of names, but you have to spell them out:

for v in foo bar "foo bar"; do echo ${v}; done;

* If a value contains blanks it has to be quoted ("foo bar")
* Sometimes the placeholder has to be in curly braces ${v}

Just toy around a bit to get the hang of it. I am not a bash scripting expert myself, but I often find it rather useful. Here is a very good documentation: http://tldp.org/LDP/Bash-Beginners-Guide/html/