This is the 1523rd most frequent Russian word.
полным
"Полным" means "full" or "complete" in English.
Here, 'полным' is used together with 'радости' (genitive case) to indicate being filled with a non-physical quality.
Он вернулся домой полным радости.
He returned home full of joy.
In this sentence, 'полным' is used to express that the park was crowded or filled with visitors, emphasizing a state of being full.
Парк был полным посетителей.
The park was full of visitors.
This usage of 'полным' conveys a sense of completeness or entirety, describing the noun 'успехом' (success) in its instrumental case.