The addition of the prefix "с" doesn't change the meaning of the verb, it changes the aspect from imperfective into perfective. https://en.wikibooks.org/wiki/Russian/Verbal_Aspect

David, the most basic form of a verb doens't have prefixes or suffixes. Usually (with a few exceptions like купить) it is imperfective. Then you can add a prefix to it. Our prefixes are a bit similar to English phrasal verbs. A prefix adds its meaning to the verb and also it makes the verb perfective. It can modify the meaning in different ways: 1) predictable and regular, like prefix до- (it usually means: complete something you have already begun doing). 2) not very predictable... when the combination "prefix and verb" took on some "specialized" meaning and we aren't sure why this one. This meaning may not be fully predictable, but at least it is in line with prefix's own meaning, usually. 3) It can just make the verb perfective, without really changing the meaning (apart of the shades perfectives normally have). In other words, among many prefixed forms there is usually one whose meaning is not very different. Alas! For different verbs it can be different prefixes:( For "смотреть" it is "посмотреть". For "делать" it is "сделать". c- can mean: - just perfective with a shade of result/completion, like in сделать - "off" (off the surface). - "with", "con-", "co-" When the prefix DOES adds something to the meaning (cases 1 and 2 above) you may want to have this prefix in an IMperfective verb. Then there is a third iteration: you add a suffix! E.g. you have делать - "to do (regularly)", "to be doing (now)". доделать - "to successfully complete something unfinished (once)" and you need a verb with meaning доделывать - "to finish (regularly) something unfinished" "to be completing now". But there is not such a form as *сделывать. No one needs it, as it would be too close in meaning to just делать.

"C" is like English Perfect in all variates. "Делаем бутерброды" - We are making "Сделаем бутерброды" - we will have made "Делали бутерброды" - we were making "Сделали бутерброды" - we have made Russians ask yourself for questions to know the form of verbs. 'What to do?' - for verbs without "C". And "What will have done?" - for verbs with "C".

Imperfective Aspect я делал бутерброды 1) Incomplete, ongoing, interrupted or repeated actions 2)Actions that are repeated an unspecified number of times, or are habitually repeated are imperfective. Я делал\делаю\буду делать бутерброды каждый день 3)Expressions when you specify a length of time use the imperfective aspect. (All day, for 2 hours, etc.) (Verbs beginning with “про” may be an exception to this rule.) Я делал бутерброды минут 15. Perfective Aspect Я сделал бутерброды Successfully completed actions. The perfective aspect is used for single events where the event has been fully completed.