This one is easy to answer...

targetX + " : " + x

is doing a concatenation of strings: it takes the two variables, convert them to strings if needed (because in the + sequence, there is a string) and make one string out of them. So the " ; " is just a string: space, colon, space. No special meaning, only display purpose, because println will show the result on the console of the PDE.

println() outputs the information to the console that could be anything from a string to a variable etc.
but you can also combine both, for example when you want to print out the value of some variables, in this case targetX and x and want to format them in some way. So you can add some strings for example you could write

println("value x is: "+x); which would print out value x is: 100 when x is 100... makes it easier to read compared to just some numbers in the console.

so what your example does is combining two variables targetX and x with a ":" inbetween, wich would output something like : "200:100" to the console when targetX is 200 and x 100.
so its not really about the ":" in this case, its more about combining strings and variables by using +

Just run this, and look at the result in the console:

String targetX = "Stuff";
int x = 42;
println(targetX + " : " + x);

Often, a small experiment is better than long explanations.

ok…