The symbols mean UpTick and DownTick, respectively. They are determined in relation to the prices of previous trades (other than the last trade). Pluses and minuses are used to indicate the number of Upticks and Downticks that come in succession.

For example, the price was 100. Then, there was a print at 101: this is an UpTick, and we will see “+”. Then, there was another print at 101. The “+” will remain. If the next print is priced at 102, we will see “++”. For example, the print after that will be priced at 101. Now, we will see “-”.