In PIC24 programming, why does AD1PCFGL = 0xFFDF mean assigning analog to AN5?
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I cannot emphasis this point enough. Read the data sheet. If you don't understand, read it again. Think about what the instruction is actually doing. You will never have success working with microcontrollers if you don't understand what the SFRs control.

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AD1PCFGL is a 16-bit register according to the A/D converter datasheet. You can't fit 32 bits into a 16-bit space, so ignore the upper byte and focus on the lower one. (Your compiler / assembler is doing this for you, BTW)



0xDF = 1101 1111, so PCFG5 is being set to 0. Per the description, that makes it an analog-mode pin. I'm sure you'll be able to figure out the relationship between PCFG5 and AN5.