Are coins and cryptocurrencies synonyms?
Yes, they are. Examples are Bitcoin and Ethereum Coins, in contrast to tokens, have their own blockchains. A token is placed on the blockchain of a coin (mostly on Ethereum, like the REME token).
You should know, that tokens are digital units of account, created on the basis of an already existing blockchain or cryptocurrency. Tokens can be used for various purposes and have a versatile functionality due to the programming of so-called "smart contracts."
What does the blockchain mean?
Blockchain is a decentrally stored, cryptographically secured, concatenated list of data sets. The blockchain is, therefore, comparable to a transparent database: In a kind of "digital account statement," all transactions are recorded, can be viewed by all members of the network and cannot be changed afterwards.
As far as I understand, it provides a high level of transparency and security between the individual partners.
You are totally right. If we return to the coins question, it should be mentioned, that tokens, like coins, are usually traded on a stock exchange after the ICO. Thus, from a practical point of view, there is hardly any difference between a coin and a token for the buyer.