What does "injection current" mean in the context of a microcontroller datasheet?
Normally the only current that'll enter the pin is tiny amounts of leakage sub micro amps but, if you have a signal input that is a bit higher than Vcc or a bit lower than 0v, substrate parasitic components may take more current and it is this current that must be limited to prevent device failure or improper working.

See the notes at the bottom of the table for how much more than Vcc you can raise an input and ditto for signals that might fall below 0v. This is usually only an issue when connecting chips of differing supply voltages. Basically, if you had a 10v logic voltage you could feed it into a 5v logic input if you used a series resistor of 1k ohm or greater. This would inject 5mA or less. Take note that you can only do this to a few pins before the total limit of 20mA is reached.