CSS multiple selectors without comma

I am alittle confuse about CSS Selectors, I understand that we can group multiple css selectors with a comma if they share similar attributes, but what about multiple css selectors without commas e.g like this:

.ui-datepicker-rtl { direction: rtl; }

.ui-datepicker-rtl .ui-datepicker-prev { right: 2px; left: auto; }
.ui-datepicker-rtl .ui-datepicker-next { left: 2px; right: auto; }

Answer

When you use the comma, like

#menu, .item

you are saying:

all elements whose id is menu AND all elements whose class is item

When you nest selectors without the comma, like

#menu .item

you are saying

all elements that has class item inside a container whose id is menu