dollar cost averaging (DCA)

if you invest when its bottom and the thing goes up, you earn.
if you invest when its top and the thing goes down, you lose money.
but you cannot predict up or down.
so invest every month/week/day (sometimes it'll be top sometimes it'll be bottom but they will eventually cancel each other out).
suitable for long term investors because in the long term, everything usually always goes up.

timing the market means,
you wait wait wait and read news then suddenly you think it will go up, so you fast fast buy...
then wait wait wait suddenly you see news and think it will go down, then sell sell sell...
so you keep trying to buy low sell high based on "research" and news.
usually for traders that just wanna earn some quick money instead of investors that plan for the long term.