One of the biggest problems I’ve had in swimming is crossover in my freestyle stroke. It’s probably been the biggest contributor to me being slower than I would like.
Crossover is when the hand and arm crosses your central plane during the stroke. This makes it hard to swim in a straight line leading to a weird snaking in the water.
Here is a rather extreme example of a chap from YouTube to give you an idea.
Coach Robb (whoever he is) explains in more detail about this and how to avoid it.
I’ve been doing this for years without really understanding it or knowing what to do about it. Now I know to keep my hand entry and bit wider and make sure my hand isn’t crossing over my centre line at any point – keeping my tracks wide like I’m ‘hugging a tree’ as demonstrated below.