The phrase "successful spell" means a period of time when something or someone achieves their goals or achieves something positive.
Sentences with «successful spell»
Both had long and largely successful spells in office, during which they changed the nature of politics in their countries. (politics.co.uk)
I have run a very successful spelling bee at my school for over 5 years now. (tes.com)
Following a highly successful spell with Shakhtar, Henrikh then brought his talents to Germany and Borussia Dortmund, where he won a DFL - Supercup (arsenal.com)