På hösten means "in the autumn" and you use it to talk about autumn in general, usually with the present tense. To talk about the autumn of a specific year, past or future, you can say hösten + year, for example jag gifte mig hösten 1993 = I got married in the autumn of 1993. There are lots of compound words linked to höst, such as höstmys (autumn cosiness), höstrusk (autumn weather) or höstlov (autumn break), which these days is often called läslov (reading break) too.