When drilling a hole using a hand drill, unless you have a guide of some sort the hole isn’t going to be 100 percent accurate and 90 degrees (straight up and down.) Much of the time, especially in thinner wood this isn’t going to matter.
But when you do need precision, and you are using a hand drill, it can be difficult. This holds true especially if you have thicker lumber.
This video shows the way to make a 90 degree hole If you don’t have a drill press or you only have one and it is tied up. It is a great little trick from Izzy Swan: