What it means
It reflects your broker's feed only. Currency futures on CME show real exchange volume.
In practice
Spot forex has no central exchange, so nobody knows the total amount traded. Platforms count how many times the price changed instead, and busy periods with many changes tend to be the periods with the most real trading.
Traders use it the way they use volume: to compare one candle's activity with the candles around it, and to see whether a breakout came with more activity than usual.
What to watch for
Compare tick volume only within the same broker and the same pair. The numbers are not contracts or money, and two brokers will show different figures for the same candle.