文件很多这种写法，content后面的斜线数字具体代表什么意思？
.glyphicon-asterisk:before {
content: "\2a"
}
.glyphicon-plus:before {
content: "\2b"
}
.glyphicon-eur:before, .glyphicon-euro:before {
content: "\20ac"
}
.glyphicon-minus:before {
content: "\2212"
}
.glyphicon-cloud:before {
content: "\2601"
}