Hiragana are like letters. They don't have any meaning on their own, they just represent sounds. あ is "a" and か is "ka". あか is read as "aka" and is a word meaning "red". If you take the same あ character and follow it with お (o), you get the word あお (ao) which means "blue". I hope this helps.
They are a syllabary since each character stand for an individual syllable or in Japanese the writing system as a whole are 仮名 "kana"
Hiragana 平仮名 - ひらがな the primary more cursive syllabary used for native Japanese words and grammar - The kanji 平 meaning "ordinary, common"
Katakana 片仮名 - カタカナ the secondary more angular looking syllabary used for loan words or emphasis - 片 meaning "imperfect, incomplete, side"
Then you have Kanji 漢字 (lit China-character) which is the writing system adopted from Chinese used in writing most noun, adjective and verb bases. Each kanji can have multiple pronunciations depending on context. These pronunciations in a dictionary will typically be written in Hiragana for their native Japanese "Kun-yomi" (lit. meaning-reading) and Katakana for their Sino-Japanese "On-yomi" (lit. Sound-reading)
あ is a kana, specifically a hiragana, for the syllable "a"
A very good answer; that being said, technically, a "letter" is not necessarily restricted to belonging to an alphabet. You can very well call kana a "letter" (文字【もじ】), or even a whole kanji. If you want to be more specific, then you can specifically say "kana," "syllabogram" or whatever strikes your fancy.
If it's a "write this in English" question, you can click the Japanese and it will open a dictionary-style entry for the word that should give you the answer.
If it's a multiple choice question, you can check all the options and see if there are words you've already learned, which you can eliminate, but sometimes you just have to guess.
With any kind of question, if you get it wrong, it shows you the correct answer, and then the question comes back up at the end of the lesson. Now that you've seen the answer once, you can see if you've remembered it. If you get it wrong again, it shows you the answer again and the question comes up again until you can get it right.
Each kana represents a syllable, these syllables are combined to create words, much like the individual letters of the alphabet in English.
Some words can also be just a single syllable, just as "i" is both a letter as well as a first-person pronoun and "a" is the first letter in the alphabet but is also a singular article, etc.
Adding another syllable changes the words meaning just as adding another letter would create a new word in English. "a" to "an" to "ant" to "anti" to "antique"