SMA15 and SMA20 differs only in the period used for calculation. SMA15 use 15 periods or 15candles to do the calculation while SMA20 use 20 periods. so the line drawn by the SMA using 15 periods and 20 periods will be slightly different from one another and sometimes when the market become highly liquid the two lines drawn using 15 and 20 periods will cross one another and trader use it as signals to enter a trade