@Seninha Kind of, but there's no black-and-white distinction between content and functional words, only a large continuum. All words depend somewhat on context for their meaning, though some are more dependent than others. Every functional word originates from a lexical word historically; its meaning is slowly bleached over time through the process of grammaticalisation (see Hopper and Traugott's book for a standard treatment of the topic).

The preposition class in English is a case in point: It's pretty darn clear what 'inside' means, even without context, but it's much less clear what 'of' means, and then you have in-betweens like 'for' or 'to'.