I'm having a bit of trouble understanding the blue dotted lines in the following picture of autocorrelation function:
Could someone give me a simple explanation, what they are telling me?
The lines give the values beyond which the autocorrelations are (statistically) significantly different from zero. Your ACF seems to indicate seasonality. I recommend Forecasting: Principles and Practice by Hyndman & Athanasopoulos, which is freely available online. (You can also buy a paper version.)
It looks like seasonality (of length 18 periods) and a longer cyclical term of about 6 seasonal intervals.
It might also be caused by an actual periodic function
What does the PACF or IACF look like?
Edit: The plot looks to be one generated in R; the blue dashed lines represent an approximate confidence interval for what is produced by white noise, by default a 95% interval