This back-to-basics guide from Partners Measure.Monks provides an explanation for Lags Leads, as well as useful further reading.
Marketing’s impact on sales does not always occur at the time of the activity – sometimes there may be a leading effect where we see an impact before the marketing activity ( i.e. a consumer may know a promotion or new product update is imminent so sales drop in advance), or a lagged effect which occurs sometime after the marketing ( e.g. where sales occur the following week or where you are promoting ahead of stock availability)