What does it mean to “skip two spaces” in a document/word?

The modern way to paragraph is to press the return or enter key twice at the end of one paragraph before starting the next one. That puts an extra space between paragraphs. That is done now in many places instead of indenting the beginning of a new paragraph. 

Space bar, not enter.  The space bar gives a blank space, so if you want two, you just tap that key twice.  No letters for two spaces, you skip two spaces before giving a new letter.

Pressing enter usually adds a line feed (starts a new line).  That is not a blank space, it is a line skip (an empty line if you just press enter before typing anything).

Letter spacing is not the same as line spacing.

These days, text-writing programs have a lot of automated systems so the program will fix things a certain way and you don't much have to think about it.  You can tell a program whether you want sentences to be separated by a single space or double space (skip only one space after the period, or two spaces after the period).

You can set the line spacing between paragraphs to make it so you do not have to actually press enter multiple times, but instead when you press enter, the text will migrate down a fixed distance (single line, 1.5 lines, double lines) or whatever.

These are all style choices.  Some people follow the "one space after a period" rule, and others follow the "two spaces after a period" rule.  Choose one, or the other, but don't do one some of the time and the other some of the time.  It will look "wrong" somehow if you do.

Just be happy that you never had to use a real typewriter.  Big pain in the butt.