The tide is the daily rise and fall of the sea level. You can count on the regular changing patterns of the tide, unless of course a tsunami is headed your way.
In a figurative sense, tide can mean a rising swell of support or unified feeling about a certain opinion or topic. So you may surf at high tide but also enjoy the rising tide of enthusiasm about surfing. If there is a wave of anything — immigration, inflation, terror — it can be referred to as a tide. No surfboards can navigate those tricky waves.