Tidal currents ebb and flow with the tide. The movement of the tide near the shoreline going up at high tide and dropping down at low tide create tidal currents. So like the tide, tidal currents are affected by the Earth, sun and the moon. Of course, we tend to only talk about the moon affecting the tide which is mainly because, as it is closer, it exerts a greater force. Tidal currents will also increase and decrease depending on the phase of the moon. When it is a full moon or a new phase, currents will be strongest (spring currents), whereas when the moon is at 1st or 3rd quarter phases, currents will be weakest (neap currents).
Tidal currents are strongest in areas with strong tides. Certain areas have bigger tidal currents than others. Those sailing around the English Channel would of course need to know about these currents, whereas someone sailing in the Mediterranean would not be severely affected. Inside the TZ Navigator v3 and TZ Professional v3 UI you can easily display the direction of currents and by clicking on individual currents, you can find the speed and direction which change with the tide.