Your 'catch' is right at the front of the stroke as you tip your hand down and bend your elbow to get hold of the water:
A good catch action presses the water backwards, which propels you forwards. The catch is critical in swimming because if you fail to get a good hold of the water at the front of the stroke you lose most of your propulsion through the rest of it.
Instead of pressing the water backwards, as we see Mr Smooth doing above, many swimmers press down on the water - or even worse, drop their wrists and push forwards! If you've visited Swim Types and discovered you're an Overglider then it's very likely you drop your wrists like this under the water as you stretch forwards as far as possible. We call this 'putting on the brakes' :
If you drop your wrist then you'll feel the water striking the palm of your hand as you travel forwards. This creates a pressure on the palm but is only a braking force, not a propulsive force. If you push downwards on the water you'll also feel a strong pressure on the palm, as water is heavy and resistant to movement. The problem with pushing down is that it fatigues the relatively weak muscle groups of the shoulder and gives you no propulsion:
Now here's the thing, many swimmers are under the impression that when they have a good catch they'll feel a strong locked-on sensation or a solid connection with the water. By pressing downwards, or dropping the wrist and pressing forwards, they feel pressure on the hand and perceive this as a good catch. Do you feel this when you swim?
When you improve your catch technique and start pressing the water backwards, you actually feel less pressure on the palm because you're helping the water on its way past your body. If you are looking for a strong connection with the water then pressing backwards will probably feel wrong at first and you may have backed away from this good technique in the past. This is perhaps the biggest reason why a good catch is so elusive.
To improve your catch, focus on tipping your wrist downwards, as shown by Mr Smooth above and bend the elbow early on, in front of your head. This will help you press the water backwards. While you develop this technique don't worry about feeling 'latched on' or 'anchored' on the water at first, this will come in time
Many Overgliders appreciate that they need to lift their stroke rate (strokes taken per minute) to swim faster. However, this is extremely hard work with a dropped wrist in the stroke - it's akin to driving a car with the handbrake on. By removing the braking action your stroke rate will naturally lift up without any increase in effort.
One final note: A great catch is technically very hard to achieve and is what separates great swimmers from merely good swimmers. However, to make big improvements in your speed in the water you don't need a perfect catch - just by starting to press the water backwards you'll achieve some nice gains in your speed and efficiency in the water.