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 +