What is the Coin Base Text of a bitcoin block? On the site https://coin.dance/blocks#blockDetails, there is a column Coin Base Text with values like Mined by AntPoolk9 Z4,b or *3Zc/BTC.COM/mm.j΀n~OoBրn*e*@ for example. What does it mean? Is it equivalent to the nonce (because I thought, the nonce is always numeric)? 

And where is this information stored? In each bitcoin block? And why?
Every block has one coinbase transaction, which is a transaction that miners use to send the block reward and fees to one or more outputs.

Since that transaction is generating new coins it has a special kind of input, that is called the coinbase, which allows up to 100 bytes of arbitrary data to be written to the blockchain. Most mining pools will put some sort of text in that field to identify themselves as the miner of that block. A solo miner can put anything* they want in that field, such as the message that Satoshi Nakamoto put in the Genesis Block coinbase:


  The Times 03/Jan/2009 Chancellor on brink of second bailout for banks


*As of block 227,836, the first few bytes of the coinbase must contain the block height