One of the meanings of the stem пол- in Russian is “one half.” The place we most commonly see it is in phrases like “It is half past one” (see details) or “I was there at half past one” (see details), but it can combine with other nouns as well that have nothing to do with clock time. The second noun shows up in the genitive singular form to make a single new word:

There is a quirk of spelling in regards to words that start with this stem. If the second part of the word starts with a vowel or with л, then you are supposed to write it with a hyphen:

Likewise you should use a hyphen if the second word is a proper name:

Otherwise the words are not hyphenated:

Words starting with пол- are most often found in the nominative and accusative cases, and in literary Russian the accusative of these words always copies the nominative. You usually don't have to worry about the other cases. Actually, a really good student of Russian will immediately ask, “But how would a Russian deal with those words in the other cases?” The answer is “inconsistently.” The rules of proper writing say one thing; conversational Russian often produces other forms. As a beginner it's best to stick with using them only in the nominative and accusative. If you have to talk about half of something in any other case, substitute the word половина instead.