What does a square symbol mean ?

[Richard]

Those are special UTF-8 symbol codes. UTF-8 is a standard for text encoding, which is basically a huge table of all possible letters, characters, modifiers etc. which exist in the world (and then some). Each letter of text is assigned a number in this table. (The letter "a" for example has code 97). I think either that persons' phone or twitter messed up somewhere, and did a wrong letter-conversion. It could be that that person wanted to tweet an obscure symbol (to make a smiley or something) and it got lost somewhere in the conversion.