To have an effect is to produce a change in something. Two spoonfuls less sugar, and after a month the tea tastes different.
It takes on, never to or in: the rain had an effect on the harvest. Effect is the noun and affect is the verb, which is the pair English speakers get wrong at about the same rate as learners do — the rain affects the harvest, and it has an effect on it. A knock-on one travels down a chain, and to no effect means nothing changed at all.
1phrasehave an effectTo produce a change in something.
“Small changes have an effect over a year.”
have an effect is a causing of change.
Also accepted in games: make a difference
Practised in Common Word Pairs