Numbered cards/aces can only be played at the end of one of your caravans. Face cards/jokers can be played on any card in your caravans OR your opponent's caravans.
Continuing the numerical direction means up/down. If your cards go 8 6 5, your next card can be 4, 3, 2, A of any suit. Alternately, you could play a higher card of the same suit. For example, if the 5 were the 5 of spades, you could play a 6, 7, 8, 9, or 10 as long as it were a spade. You can reverse the direction of a caravan by playing a queen on the last card in the caravan. That caravan also then takes the suit of the queen.
Caravan is a non-casino (some might say anti-casino) game that was invented by caravan guards who lacked complete/matching decks. They turned that fact of life into a core element of the game (deck building).