We call an en­cod­ing $$E(m)$$ of a mes­sage $$m$$ “in­trade­pen­dent” if the fact that $$E(m)$$ en­codes $$m$$ can be de­duced with­out look­ing at the whole en­cod­ing. For ex­am­ple, imag­ine that you know you’re go­ing to see an 8-let­ter English word, and you see the let­ters “aardv”. You can then de­duce that the mes­sage is “aard­vark” with­out look­ing at the last three let­ters, be­cause that’s the only valid mes­sage that starts with “aardv”.
In an in­trade­pen­dent en­cod­ing, some parts of the en­cod­ing carry in­for­ma­tion about the other parts. For ex­am­ple, once you’ve seen “aard”, there are $$26^4 = 456976$$ pos­si­ble com­bi­na­tions of the next four let­ters, but “aard” cuts the space down to just two pos­si­bil­ities — “vark” and “wolf”. This means that the first four let­ters carry $$\log_2(26^4) - 1 \approx 17.8$$ bits of in­for­ma­tion about the last four. (The fifth let­ter car­ries one fi­nal bit of in­for­ma­tion, in the choice be­tween “v” and “w”. The last three let­ters carry no new in­for­ma­tion.)