"The lunch is tasty."
Translation:Obiad jest smaczny.
Can somebody link me to where it says the difference between smaczny and smaczna? From what i know now it can only presume it's random number generated.
Each noun in Polish has its grammatical gender: male, neutral or female. Adjective has to fit its form to the gender and the case of the noun it describes. "Obiad" (lunch/dinner) is male, so the adjective has to be in male form "smaczny obiad". "Zupa" (soup) is female, so it would be "smaczna zupa". "Mięso" (meat) is neutral, so it would be "smaczne mięso".
Because the trick with using "to + noun in nominative" instead of "jest + noun in instrumental" can be done only in structures "noun is noun" 
See my comment to 94BlueLane's question.
 To be more precise, it can be also done with other words or groups of words, as long as in a sentence they perform the function of subject as a noun or a noun phrase. Perhaps also the function of object, but I can't think of any case where no "real" noun would be used as a part of object.
I was under the impression that "jest" governed a certain case (instrumental?) and so any following adjectives should end in "m/em" or something like this. Can someone explain this for me? :)
This is true for phrases type Subject-Verb-Object, where object can be either a noun, or a noun described by some adjectives (noun phrase). In that case, the verb "jest" governs the object and requires instrumental case:
- Pies jest zwierzęciem. - The dog is an animal.
- On jest dobrym człowiekiem. - He is a good man.
The above is a different case: Subject-Verb-Adjective. We do not have an object (or another noun besides the Subject), we only use the adjective to describe the Subject. In this structure there is no instrumantal case, only the adjective in nominative:
- Pies jest miły - The dog is nice.
- On jest dobry - He is good.
Yes, "obiad" is masculine. BTW, "śniadanie" (breakfast) is neuter, and "kolacja" (supper) is feminine.
Are most words ending in consonants male, ending in a female, and the rest neuter?
Roughly, yes - but you will find more details here: https://en.wikibooks.org/wiki/Polish/More_on_nouns_-_genders
Could someone explain why isn't it possible to use to instead of jest in this case?
You can only use the 'to' construction in an "X is Y" sentence in which both X and Y are noun phrases.
You can think of it as something like a "=" sign. You can say "The lunch = a tasty meal" but not really "The lunch = tasty".
More info here: https://www.duolingo.com/comment/16373167