You learn quickly that when cutting something freehand, (without using a guide on the saw), it’s very difficult to stay on the line. As you wander off the line the blade that is still in the wood you’ve already cut will hold the blade in the direction you’ve drifted off. If you try to get back on the line you will only be able to change the direction of the cut by a very small amount. And it takes quite some distance before you get back to the line. Then the tendency is to over-correct and you go off the line in the other direction, so your intended straight line cut weaves back and forth somewhere near the line on both sides, but not on the line.
One solution is that when making a free-hand cut, cut so there is extra, and then do a finish cut on a table saw with a guide.
Another solution is to lift the blade out of the cut and drop it back in, on the line. And go from there.
There’s a better solution: Stay focused and correct quickly. The problem occurs because you’re too far off the line before you correct the direction of your cut. If you’re paying close attention, you can make tiny adjustments the instant you see the blade drifting. On the cut that I did in the picture I was able to see a change in direction before the blade had moved even a 1/64th inch off the line. Then with an equally tiny 1/64th inch correction I stayed on the line. When I finished it was a nearly perfectly straight cut.
I notice the same thing in my life. Stay focused and make small corrections quickly, before they cause bigger problems that are long and hard to correct. It makes life a little bit smoother.