Altcoin is also known as alternative coin (specifically to bitcoin); these are coins which have variable values when traded to bitcoins or fiats. You might also want to know their features - well, each coin has unique features which you have to google yourself because they are too many to be discussed. Their features attract investors and that causes their price to move, however, there are some altcoins that were created without any legitimate purpose. You might want to research about pump and dump in this forum too.

I hate not to explain to you everything about altcoins but I guess doing your own research would be beneficial.