Many toilets have a single hole at the bottom of the toilet bowl (Siphon Jet Hole) which is a key part of a toilet flushing properly. Water should spurt through this hole when you flush the toilet pushing the water out the large opening to flush. This hole can become clogged with calcium and mineralization which impedes the flow of water causing problems with flushing (other toilet styles have this large hole at the rim which serves this same function).

Someone else has a video on the same issue
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This article has a good diagram of the toilet showing the siphon jet hole (the article is explaining how to clean the rim jets, which are different than the siphon jet). Note that sometimes this "hole" may be set back in some toilets and not even obviously visible looking into the bowl.
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The following are other potential flushing-related issues you can have with a toilet.

This youtube video explains how to clean out the jets around the rim of the toilet, which can be a source of flushing problems for a toilet as well.
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CLR (brand) Calcium, Lime, Rust Remover is one commonly available product that can be used to clean the small jet holes.

There are also issues with the mechanics in the toilet tank that can cause issues with flushing. This video has a good overview of the typical issues you might face.
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This isn't about flushing a toilet, but if you hear a hissing sound when your toilet is not flushing and you have a Korky Toilet Flush valve your problem might be from a worn rubber pad. This video explains the easy fix (it worked for me):
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