내디면 is one of those bogus words that don't exist in standard Korean.
It seems that a strong modern trend is to make things short at all costs, and people go to great lengths to do that.
So in addition to the original 내디디다(step/tread forward) and its contracted version 내딛다, some use 내디다.
내디다 (unrelated to 내다) is not a standard word, and I haven't seen a dictionary that lists it as a word.
But that doesn't stop people from using it. If you google it, there seems quite a bit of use already.
So it is only natural that you couldn't figure it out.
Strictly speaking, it is not a word - don't use it in a test for example.
As a side note, the language standards state that 내딛다 (shortened version of the original 내디디다) may only conjugate with endings starting with a consonant (내딛고, 내딛는, 내딛더라도, 내딛지만,...) and not with those starting with a vowel. This is to avoid proliferation of multiple spellings (내디뎌도 v 내딛어도, 내디뎠다 v 내딛었다 for example). So 내딛으면, 내딛어도, 내딛었다, 내딛을 are all wrong according to the standards. 내딛다 is a "restricted" form in this sense, but some people not only use it like any other normal words but even go further and invent a clever form like 내디면. It is an example of how languages constantly evolve.