Way back in the Dark Ages, I wrote a piece all about Second Life shoes. Due to recent changes in the way Second Life works, I need to add a small addendum to that.
Invisiprims are a special sort of prim. They use a script to make the avatar mesh inside of, or behind them, invisible. Invisiprims used to be the standard way that shoe makers would hide the unsightly part of your foot that would otherwise be visible below the sole of a pretty high heeled shoe.
The trouble is, invisiprims were always a bit of a hack, making use of a flaw in the Second Life code. When Linden Lab enabled the viewer to see Mesh objects, they also fixed this loophole. The result is that anyone using a current mesh-capable viewer (and that is just about everyone, nowadays), who is ALSO using the Advanced Lighting Model and Shadows in their graphics preferences, will not see your invisiprims. On their monitors, your feet will look like ugly clubs.
There are a lot of shoes out there, especially freebies, that still use invisiprims. Be sure that your shoes come with an alpha mask item...in your inventory, it will have the symbol of a shirt with a grid texture on it. Wear this alpha mask item with your shoes. If you are wearing any other alpha masks, such as for a Mesh dress, use the Add command, not Wear, to add the shoe alpha mask to your ensemble.
Some shoes, like boots, can be fixed. Even if you can't remove the invisiprims (most shoes are No Modify), you can add your own alpha texture to hide your foot. This will not work with shoes that show part of the avatar foot, like pumps or sandals. You can find collections of foot alpha masks on the Marketplace, see the links below.