Here's a definition. Don't be put off by its apparent complexity: I think it's simpler than it looks.

Effet de base :
Dans l’analyse conjoncturelle on explique souvent l’évolution des taux de variation annuelle d’une variable par des « effets de base ». On est en présence d’un effet de base lorsque l’évolution du taux annuel d’une variable d’un mois t au mois t+1 varie non pas en raison d’une variation du niveau de la variable du mois t au mois t+1, mais plutôt en raison de l’évolution du niveau d’il y a 12 mois.

Essentially, it means shifting the baseline, taking a longer-term view rather than measuring progress over short periods. So your business may have had a really bad year if you use January 2015 as your baseline, but it may be doing very well if your baseline is 2008.

And here's Webster's:
Baseline
A line serving as a basis; especially : one of known measure or position used (as in surveying or navigation) to calculate or locate something
A usually initial set of critical observations or data used for comparison or a control