Arbitrage is buying an asset for a lower price in one market and selling it for a higher price someplace else, or later on. Finance guys go for this all the time (see here to read about it, along with the caveats for you, the average person: https://www.investopedia.com/articles/trading/04/111004.asp

With cryptocurrency, you are looking for coins that tend to move in price at the same rate -- but opposite (or inverse, as we say in statistics). In other words, if Bitcoin drops while Litecoin rises (and it has for me), then it's good to buy Bitcoin when Litecoin is up. Why? Because you'll buy Litecoin with your Bitcoin when Litecoin drops.

The tricky bit is finding a marketplace the doesn't take too high a fee for the transfer, which is one of the reasons that I don't arbitrate Bitcoin right now. My best bet at the moment is Robinhood (https://join.robinhood.com/marianp-f74cb1), where my trades are free, but I have to trade from cash and back to cash from Crypto. But there is no -- I mean 0 -- transaction cost. 

We'll talk about coins that move against each other in future posts.