If you are a IO designer, you should know a lot of analog knowledge and ESD latch etc.
If you are just a user. What you should learn is just the basic characteristics of the IO. For example, the size, pad-pad interval, function description, driving ablity, voltage etc. Read the IO library user guide from the foundary carefully before use of it. They will tell you the characteristic and how to use it.
yes if you want to design IO by yourself , first you shold konw some analog knowledge, layout design method is also needed, ESD and ESD protect circuit and Latch are necessory, if the foundry are detemined, normally foundry will recommend you the ESD protect circuit structure and detail design parameter, such as, mos length,width, resistor, cap and so on.
if you only want to use, you can read IO library mannualy, that is enough