Tides are the periodic fall and rise of sea-level due to gravitational forces of the moon, sun or earth-rotation. When asking questions specifically about tides, use this tag and also include the [ocean], [moon], [sun] or [earth-rotation] tag if appropriate.
When Earth, moon and sun are approximately lining up (full or new moon) gravitational forces are added up and tides are stronger than usual. You call this phenomenon spring tides. When Earth, moon and sun form a right angle (first or third quarter of moon) gravitational forces partially cancel out each other and tides are weaker than usual. You call this phenomenon neap tides.
(Don't mix them up with solar tides in the atmosphere, which are inertia-gravity waves caused by the absorption of solar radiation in the the atmosphere.)