To me the phrase
prohibited l’chatchila seems a bit confusing. Can someone explain what this means in plain and simple english?
(context: someone says "action X is prohibited l’chatchila")
"L'chatchila" means "from the outset", meaning that before one did action 'x' the halacha was that it was forbidden. However, if one was not familiar with that halacha and did action 'x' without knowing that there was a problem, then "b'di'eved" ("after the fact") the halacha might be different, meaning the consequences of what was done would change.
In practice this means that if one goes to their LOR and says "I did this, now what?", the Rabbi might say: "L'chatchila that was forbidden, but b'di'eved, since you didn't know, its not as big of a problem (or whatever the halacha b'di'eved is)".
For actual halachic examples this website gives one:
L'chatchila — a priori. Refers to the ideal halachic condition. L'chatchila we don't prepare pareve food in a meat pot if we intend to eat the food with milk.
B'dieved — a posteriori. Refers to an acceptable halachic position but one that is short of ideal. B'dieved pareve food that was prepared in a meat pot that was mixed with dairy may be eaten.
For actual questions of practical halacha always CYLOR.
L'chat'chila means, as HodofHod pointd out, "from the start". That is, when you're examining the permissibility of the action before doing it, you should consider it prohibited.
Now, as far as fulfilling a requirement by doing an action goes, we (very often) say that what is okay not l'chat'chila is also okay in a situation of difficulty, where one cannot easily fulfill the "l'chat'chila" method. For example, if it's required to stand while fulfilling some mitzva (command), but, after the fact (i.e., not l'chat'chila), one's considered to have fulfilled it even if he did so sitting, then we'd usually say that someone can, from the outset, sit while performing this act if standing would be difficult. Exactly what qualifies as "difficult" should be asked of a competent, orthodox rabbi.
So likely someone who says "prohibited l'chat'chila" is transferring that extra connotation from the realm of actively fulfilling commands to the realm of prohibitions, and means "prohibited except in a situation of difficulty" (where, again, "difficulty" would need to be assessed by a rabbi). But it'd be best to confirm that meaning with whoever is using the phrase.